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WRAK
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Reged: 02/18/12

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How good are reflectors for resolving binaries?
      #6022897 - 08/13/13 01:12 PM

At the relative high cost for a 225mm iris diaphragm I learned the obvious that large CO's make a scope less suitable for resolving doubles - especially if the CO gets larger than 0.35.
So far I have the impression that Newton's and Mak's with their relative small CO of less than 0.2 up to 0.28 are of good use for resolving binaries and even benefit a bit from the reduction of the Airy disk as byproduct of the CO.
SCT's and similar constructions with rather large CO's of above 0.3 or even 0.35 seem to be not this good instruments for this purpose. I even have the suspicion that you need rather perfect seeing to resolve doubles near the Dawes criterion - not only because of the larger aperture compared with a refractor but also due to the large CO.
I would like to learn the contrary - did you make any observations with a SCT around Dawes?
Wilfried


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Erik Bakker
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6023038 - 08/13/13 02:05 PM

Mak's are roughly the same with respect to CO as SCT's.

Only MakNewt's and Newt's are widely available with CO's under 20%

Big CO's are not very good for clean splits. They can help a bit under very good circumstances with equal doubles though.

I enjoy good refractors and my 17% CO Newt a lot for doubles.


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David Knisely
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6023211 - 08/13/13 03:13 PM

Quote:

At the relative high cost for a 225mm iris diaphragm I learned the obvious that large CO's make a scope less suitable for resolving doubles - especially if the CO gets larger than 0.35.
So far I have the impression that Newton's and Mak's with their relative small CO of less than 0.2 up to 0.28 are of good use for resolving binaries and even benefit a bit from the reduction of the Airy disk as byproduct of the CO.
SCT's and similar constructions with rather large CO's of above 0.3 or even 0.35 seem to be not this good instruments for this purpose. I even have the suspicion that you need rather perfect seeing to resolve doubles near the Dawes criterion - not only because of the larger aperture compared with a refractor but also due to the large CO.
I would like to learn the contrary - did you make any observations with a SCT around Dawes?
Wilfried




I managed to follow the closest approach of Porrima when it was under an arc second separation in my 9.25 inch SCT (36% central obstruction BTW). It didn't resolve the pair for the two years (2004, 2005) around closest approach, but it did show elongation during at least some of that period. In any case, the SCT does just fine on many double stars down to around Dawes Limit, as long as the fainter star isn't really faint and doesn't sit right on top of one of the diffraction rings. Clear skies to you.


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PJ AnwayModerator
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6023615 - 08/13/13 06:01 PM

I'm not a fan of "large-CO" scopes for double star observing. That is why I built my newtonian reflector with a 16% CO. It's excellent for doubles, but I must say my preference is still refractors.

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Ed Wiley
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Reged: 05/18/05

Loc: Kansas, USA
Re: How good are reflectors for resolving binaries? new [Re: PJ Anway]
      #6024263 - 08/14/13 12:13 AM

Read Taylor in Chapter 11, p, 118 in Observing and Measuring Visual Double Stars (Argyle, ed). The last paragraph on that page begins:

"The effects of central obstructions often alleged to degrade imaging quality of reflectors quite seriously compared with that of refractors, can be similarly dismissed."

He then goes on to saw why.

I do my serious imaging and measuring with both a DK and a SCT. When collimated they do the job just fine. Will they reach the Dawes limit for the aperture? Yes for the SCT as judged by tests at the TSP last spring. I haven't tested the DK yet but results to be published for doubles down to 1" compare well with interferometric measures made by pros with much bigger apertures. Reflectors have nothing to whine about relative to refractors, especially considering the size of an 11" refractor compared to an 11" SCT.

Ed


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: How good are reflectors for resolving binaries? new [Re: Ed Wiley]
      #6024427 - 08/14/13 03:00 AM Attachment (34 downloads)

Quote:

Read Taylor in Chapter 11, p, 118 in Observing and Measuring Visual Double Stars (Argyle, ed). The last paragraph on that page begins:

"The effects of central obstructions often alleged to degrade imaging quality of reflectors quite seriously compared with that of refractors, can be similarly dismissed."
...
Ed




This may be true for personal experience with specific amounts of CO. But I know for sure that excessive amounts of CO of 0.4 and larger lead to may be nice images of stars with 10 or more diffraction rings but with this much loss of energy in the Airy disk in favor of the diffraction rings fainter companions get simply lost. And even for not this faint companions the required aperture gets much larger compared to a CO of 0.
For my C925 the Dawes criterion would be about 0.5" and so far I did not even come close to this value (despite optical tests stating a resolving power of 0.55") but I will try again.
Wilfried


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azure1961p
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6024557 - 08/14/13 07:28 AM

It was my strong understanding that at higher frequency resolution a larger CO helps on Dawes seperated doubles by slightly reducing the central disc via putting that energy into the rings. This precludes a secondary star getting masked or obliterated by a diffraction ring since the spurious discs are merged or overlapped so that this all takes place within the rings. I have t as yet tried for a Dawes split with my C6 but Porrima for example looked text book, if a little bright ringer at 38% CO.

Pete

Edited by azure1961p (08/14/13 07:30 AM)


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fred1871
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Reged: 03/22/09

Loc: Australia
Re: How good are reflectors for resolving binaries? new [Re: Ed Wiley]
      #6024756 - 08/14/13 09:59 AM

Ed, your quote from Taylor (above) is fair but can be misleading. I think people need to read his whole paper on the subject of why reflectors can be fine for double stars.

And take into account his note, near the end of his chapter:
"Unequal close pairs are much more difficult than equal pairs at the same separation, especially in reflectors generating accentuated diffraction rings..."

His further comments there need to be looked at as well, including his mention of the influence of seeing and other factors, producing "altogether a more complex affair [on unequal doubles] than the corresponding questions for equal doubles and their observation consequently yields much less reproducible results."

I think we're seeing plenty of evidence of that last point in various discussions here on the difficulty of various unequal pairs.


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Ed Wiley
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Reged: 05/18/05

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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6024900 - 08/14/13 11:28 AM

Fair enough, Fred, thanks for pointing this out. However, apparently this has nothing to do with central obstructions. In Taylor's case, if I understand it, this is due to the fact that his primary was undercorrected (p. 136) An undercorrected primary, according to Taylor, creates "residual spherical consequently tending to give rise to defraction rings of largely enhanced intensity." It is under this particular case that the reflector might be inferior to the refractor in the case of equal pairs and not the fact that the reflector has a central obstruction. At least that is how I understand Taylor's argument. But, you bring up the interesting point of very unequal pairs. I will have to read Taylor more carefully regarding this point but the revealed wisdom from other sources seems to reinforce your point. For visual observation I suppose the only way to tell is with a direct comparison. I would love to see something empirical.

(Edited -- I meant equal not unequal pairs)

Clear skies, Ed

Edited by Ed Wiley (08/14/13 03:01 PM)


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: How good are reflectors for resolving binaries? new [Re: Ed Wiley]
      #6026458 - 08/15/13 03:58 AM

Here some empirical evidence:

- STF1733 (HIP 64762) – 4.9“DS +8.99/10.37mag:
With 140mm APO hint of resolution with x75 and clear split with x140. Limit aperture 90mm with very faint companion.
With 235mm SCT clear resolution with x100, limit aperture 140mm (means CO of 0.64) with indirect vision.
Result: Zero CO limit is 90mm and 0.64 CO limit is 140mm

- BU800 (HIP 64797) – 7.6“DS +6.66/9.5mag:
With 140mm APO easy split with x75. Limit aperture 80mm with faint companion.
With 235mm SCT clear split with x100, limit aperture 150mm means CO 0.6. Higher limit aperture as for STF1733 suggests a too optimistic estimation of the magnitude for the companion.
Result: Zero CO limit is 80mm and 0.6 CO limit is 150mm


- STF1737 (HIP 65205) – 14.8“DS +7.85/10.31mag:
With 140mm APO: x40 hint of resolution, x75 clear. Companion faint, limit aperture 70mm.
With 235mm SCT split x100 with faint companion. Limit aperture with averted vision 130mm (means CO of 0.69).
Result: Zero CO limit is 70mm and 0.69 CO limit is 130mm


- STT266 (HIP 65725) – 2“DS +7.97/8.42mag:
With 140mm APO: x75 hint of resolution and with x140 clear split. Limit apperture 60mm.
With 235mm SCT Clear split with x100. Limit aperture 150mm with a fuzzy rod for apertures below - would have expected a "better" value here.
Result: Zero CO limit is 60mm and 0.6 CO limit is 150mm


- J749 – 2.7"DS +9.9/10.8mag:
With 140mm APO: With x200 and indirect vision hint of rod in the correct position, but no valid limit observtion.
With 235mm SCT split with x100 with indirect vision. With x335 limit aperture 170mm (means CO of 0.53) again with averted vision.
Result: CO 0.53 with limit 170mm wins against 140mm mit zero CO

- A1788 – 2.4"DS +9.78/11.35mag:
With 140mm APO: x200 and indirect vision hint for rod at correct position but no valid limit observation.
With 235mm SCT no resolution because the moon is so bright that it erases the faint companion.
Result: Both negative

- BU237 (HIP 65589) – 3.4“DS +8.5/10.62mag:
With 140mm APO: False potitive with x140, object maybe already too low in altitude.
With 235mm SCT the companion is barely to detect With x180. Limit aperture is 220mm, below this no trace of the companion is detectable.
Result: CO 0.4 with 220mm wins compared with zero CO 140mm.

Result in total: Larger aperture is a great advantage for fainter doubles but large CO is generally disastrous for resolving doubles.
Wilfried


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Asbytec
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6026607 - 08/15/13 07:47 AM

Quote:

SCT's and similar constructions with rather large CO's of above 0.3 or even 0.35 seem to be not this good instruments for this purpose. I even have the suspicion that you need rather perfect seeing to resolve doubles near the Dawes criterion...




Wilfried, at D/3 the first minimum reduces from 1.22 to 1.11, and to 1.0 at D/5 even with incoherent light where FWHM is reduced somewhat more from .515 unobstructed aperture to .501 at D/5. But, it's been stated for all intents and purposes, light in amateur sized scopes is essentially coherent. So, the effect in the field might be somewhat muddied (being more pronounced in coherent light.)

http://www.telescope-optics.net/obstruction.htm

It's interesting you're masking off the marginal wavefront. I wonder what that does to effective RMS. It might actually improve Strehl somewhat by reducing the peak aberration component (at the edge) similar to apodization. However, masking below the 70% zone of best focus might be detrimental in that the unmasked wave front carries a bit more aberration (OPL) relative to the reference sphere centered on best focus. Truth is, I don't know. But, something might be happening - for better or worse - to correction when you mask down to 40% or greater CO.

I have split 7 Tau (~.074" arc) and 31 Tau (~0.8" arc), both very close to Dawes for 150mm. The splits were fairly easy in good seeing. And elongated 72 Pegasi at ~.57" arc.

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=double&am...

However, following you're RoT, managing seemingly easy unequal pairs has proven elusive.

Quote:

"The effects of central obstructions often alleged to degrade imaging quality of reflectors quite seriously compared with that of refractors, can be similarly dismissed."



Such a statement might be misleading, but I believe it is accurate for splitting close, brighter, equal pairs at (or even a touch beyond) the Dawes limit.

For unequal pairs, the presence of a CO can be detrimental especially with Sep near the first ring. For example, 42 Ori was almost impossible for a 150mm as the companion is pretty much lost in the first ring. However, only through sheer perseverance and possibly some blind luck, I did manage to get a close (+/-10 degrees) PA estimate. It really boiled down to only rarely 'seeing' a fleeting speck of light in the undulating ring a few times at that location. So I went with it and happened to be close. It was certainly a very difficult observation that took a long time. There was one other "over performing" observation, but I cannot recall the double (higher deltaM and wider split.)

In any case, the presence of the CO is a player, IME. It allows a bit more high frequency resolution for close equal pairs and detracts from unequal pairs. Both in theory and in my own limited experience with doubles in very good seeing.

I felt like I could shave a few 10ths of an arc second off of 7 Tau's sep, maybe closer to 0.70" arc. The dark space (not black space) was enough to maybe get a little closer. But dealing with such small fractions of an arc second, we really need accurate measurements to work with, and 7 Tau is not (I suspect) the classic 6th magnitude double. It's close at about 0.5 magnitude fainter. That may have helped with the split. I'm itching to try it, anyway.


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WRAK
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Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: How good are reflectors for resolving binaries? new [Re: Asbytec]
      #6027655 - 08/15/13 05:06 PM

Norme - since when 31% CO? I remember 28%.
Wilfried


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brianb11213
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6027724 - 08/15/13 05:46 PM

Quote:

Norme - since when 31% CO? I remember 28%.
Wilfried



Moot. The difference between 28% and 31% is entirely imperceptible. It's hard enough to distinguish between 30% and 0%, when the optics are 1/10 wave PV or better, the seeing is perfectly steady, the scope is perfectly acclimatized to ambient temperature and the collimation is perfect.


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fred1871
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Reged: 03/22/09

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Re: How good are reflectors for resolving binaries? new [Re: brianb11213]
      #6028264 - 08/15/13 11:33 PM

Agreed on 28% vs 31% - but with all other factors equal, and very good, difference between 0% and 30% on UNEQUAL pairs is increasingly visible as delta-m increases and separation decreases. CO moves light out of the disc into the diffraction rings. Basic optics. Bigger CO has more effect. Small CO can be difficult or impossible to distinguish (under 20% versus 0%).

Larger CO (as in 30% or more) can help with equal pairs (reduced apparent disc size), but becomes a hindrance with the tight unequal pairs, especially as delta-m gets to 2 mags or more, and most obviously when the secondary is on or nearly on the first diffraction ring.


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Asbytec
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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6028336 - 08/16/13 12:19 AM Attachment (38 downloads)

I'm sorry, the correction was a result of better measurements of a couple of millimeters (adjusting up the silvered spot diameter.) It's difficult to lay a rule over a curved surface and get an accurate measure. So the revision comes from the measurement of the removed secondary baffle's base (outside diameter) which is smaller than the 'spot' by about 1.5mm in radius. I corrected it in my signature.

If I may, though, reducing the CO diameter from 140mm effective aperture and 52mm CO (~37%) to full aperture and the spot diameter of 46mm (~31%) did seem to show almost immediately in the diffraction images of bright stars. (Planetary is another story, inconclusive but I suspect improvement based on sketches over time. And any improvement might simply be the result of both full aperture and CO reduction.)

Again, forgive the rather crude measurements in the pic below and the tinkering with numbers. But, the image shows some reduction in the outer ring brightness. It's subtle and may be open to interpretation on observing conditions. But, I think there is a noticeable improvement in diffraction from 37% to 31%. The outermost ring is gone observing relatively bright stars, Arcturus in 8/10 seeing in this example. And the next inner ring is somewhat reduced. How that translates to observing unequal pairs is still undetermined, but there /might/ be a slight gain.

So can even a lightly smaller obstruction make a difference? I suspect it might just be enough to begin showing some very delicate improvement. Can't wait to do some unequal pair observing to see (along with Jupiter and Mars...and equal pairs.)

Edited by Asbytec (08/16/13 01:33 AM)


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WRAK
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Reged: 02/18/12

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Re: How good are reflectors for resolving binaries? new [Re: Asbytec]
      #6028531 - 08/16/13 03:47 AM

CO 0.31 instead of 0.28 is certainly no big deal - depending on the other parameters may be 1-2mm in proposed aperture. I just want to have my data set of limit observations (includes about 10 observations from Norme) as precise as possible - difficult enough with all these errors in the advertised data for binaries.
Wilfried


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Asbytec
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6028621 - 08/16/13 06:45 AM

I'd love to tackle some more beginning in November when the dry season begins. I am new to doubles, but find them beautiful and challenging.

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Jon Isaacs
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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6029843 - 08/16/13 08:29 PM

Quote:


Larger CO (as in 30% or more) can help with equal pairs (reduced apparent disc size), but becomes a hindrance with the tight unequal pairs, especially as delta-m gets to 2 mags or more, and most obviously when the secondary is on or nearly on the first diffraction ring.





That has been my experience. When working near the limit, refractors are significantly better for unequal doubles. Case in point, my C4.5 would do a good job on equal magnitude doubles, comparable to a 4 inch refractor. But a relatively straight-forward unequal double like delta cyg, 2.7" at mag 2.8, mag 6.6 was not possible.

With larger Newtonians, the issues are still there but the COs are smaller and in general they are seeing limited so the issues are not with the telescope optics.

I am looking forward to some good seeing to try out my new to me 13.1 inch F/5.5 with a 20% CO. The downside is that the mirror is full thickness but if I can get it cooled down, not such a problem here in San Diego, I should get some tight splits.

Jon


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3c_273
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Re: How good are reflectors for resolving binaries? new [Re: Jon Isaacs]
      #6032950 - 08/18/13 02:09 PM

Really Good. Mostly!

My C-8, often excoriated because of its huge secondary (2.5, 63.5mm", 0.31% of primary aperture) and field curvature (~9", 228.6mm"), can deliver exquisite images, given a tranquil atmosphere. The diameter of it's Airy disc is 1.26", and first diffraction ring is 1.7". See

http://www.telescope-optics.net/diffraction_image.htm

for how this is derived. The large secondary displaces light from the central disc to the rings, which is sometimes good. See

http://en.wikipedia.org/wiki/Airy_disk

for the gory details.

My C-8 is one of the old Ultimas, known for their excellent optics, and my telescope, with it's UO orthoscopics, leaves little to be desired in this respect. I actually use setting circles to find things with it.

On to what I've observed.

When both stars of a pair are close to the same brightness, The C-8 splits them very nicely. Especially with faint pairs. I've managed to make pairs like BU 394 (0.6" when observed), or BU 1313 (0.5" in 2010, observed in 2012) actually split or appear elongated.

What isn't so good is when the stars are around 1.2 - 1.9" apart, and the primary is 2+mv brighter than the secondary. Then the primary's first diffraction ring is right on top of the secondary, and the central obstruction is enhancing it's brightness. These are almost impossible for me to resolve.

Reflectors, with their inherently larger (both dollar and physical size wise) apertures, allow faint stars to be observed. The confusingly cataloged star L 44:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6010128/page...

might be a 12.5 - 13.9mv pair, 4" apart. I could see both stars in the C-8. My old Questar 3.5 (long since sold) would not have shown either star.


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WRAK
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Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6057558 - 09/01/13 12:06 PM

For reasons unknown to me I overlooked the obvious fact that I do not need a reflector to create a useful setup for checking the influence of CO on resolving binaries. If I make inverse masks for my 140mm refractor with different sizes of CO this is even better as I can start with zero CO - may be a bigger aperture would show more conclusive results but this way I can start now and have not to wait for additional equipment. A simple solution for fast and easy changing the CO on the fly will be required. Any suggestions?
Wilfried


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azure1961p
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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6057639 - 09/01/13 12:55 PM

Quote:

Really Good. Mostly!

My C-8, often excoriated because of its huge secondary (2.5, 63.5mm", 0.31% of primary aperture) and field curvature (~9", 228.6mm"), can deliver exquisite images, given a tranquil atmosphere. The diameter of it's Airy disc is 1.26", and first diffraction ring is 1.7". See

http://www.telescope-optics.net/diffraction_image.htm

for how this is derived. The large secondary displaces light from the central disc to the rings, which is sometimes good. See

http://en.wikipedia.org/wiki/Airy_disk

for the gory details.

My C-8 is one of the old Ultimas, known for their excellent optics, and my telescope, with it's UO orthoscopics, leaves little to be desired in this respect. I actually use setting circles to find things with it.

On to what I've observed.

When both stars of a pair are close to the same brightness, The C-8 splits them very nicely. Especially with faint pairs. I've managed to make pairs like BU 394 (0.6" when observed), or BU 1313 (0.5" in 2010, observed in 2012) actually split or appear elongated.

What isn't so good is when the stars are around 1.2 - 1.9" apart, and the primary is 2+mv brighter than the secondary. Then the primary's first diffraction ring is right on top of the secondary, and the central obstruction is enhancing it's brightness. These are almost impossible for me to resolve.

Reflectors, with their inherently larger (both dollar and physical size wise) apertures, allow faint stars to be observed. The confusingly cataloged star L 44:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6010128/page...

might be a 12.5 - 13.9mv pair, 4" apart. I could see both stars in the C-8. My old Questar 3.5 (long since sold) would not have shown either star.




I completely agree a quality SCT can be a formidable doublestar instrument. Theory (and reality) would suggest the slightly smaller spurious disc and brighter rings could actually enhance sub arc second doublestar resolution.

Pete


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WRAK
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Reged: 02/18/12

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Re: How good are reflectors for resolving binaries? new [Re: azure1961p]
      #6057836 - 09/01/13 02:59 PM

I certainly agree with the solid statement of Tom and even included his observations of BU394 and BU1313 in my data set of limit observations - but I want to see myself how increasing CO shrinks the size of the Airy disk. For example I would expect a kissing pair with zero CO to get separated with 0.25 CO.
And then I want to check the limit of CO before it becomes destructive as I have learned happens when reducing the aperture of my C9.25.
I suspect so far that nothing positive happens any more above 0.25 CO and negative effects begin with 0.35 and exactly this I want to check with reality.
Wilfried


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azure1961p
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6057949 - 09/01/13 04:09 PM

Wil,

I'm curious - your room reducing your aperture in the 9.25 SCT I'm guessing to see the effects of an enlarged CO. The reduced aperture would reduce resolution. Perhaps its best to leave aperture at 9.25" and simply experiment with CO silohuettes or masks. If Im misunderstanding you please clarify.

I think we are all in agreement however that at a specific seperation per aperture the illuminated first diffraction ring can be detrimental to fainter secondary companion visibility.

I'm clouded out here on my three day weekend off or Id be certainly doing doubles these evenings. Oh well.

Pete


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fred1871
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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6058380 - 09/01/13 10:04 PM

Quote:



My C-8, often excoriated because of its huge secondary (2.5, 63.5mm", 0.31% of primary aperture) and field curvature (~9", 228.6mm"), can deliver exquisite images, given a tranquil atmosphere. The diameter of it's Airy disc is 1.26", and first diffraction ring is 1.7". .....

When both stars of a pair are close to the same brightness, The C-8 splits them very nicely. Especially with faint pairs. I've managed to make pairs like BU 394 (0.6" when observed), or BU 1313 (0.5" in 2010, observed in 2012) actually split or appear elongated.

What isn't so good is when the stars are around 1.2 - 1.9" apart, and the primary is 2+mv brighter than the secondary. Then the primary's first diffraction ring is right on top of the secondary, and the central obstruction is enhancing it's brightness. These are almost impossible for me to resolve. ......





I'd overlooked Tom's note (given in part, above) when it appeared. Looking at it now, I see some matters needing clarification.

First, Tom quotes the diameter of the Airy disc and the first diffraction ring (1.26", and 1.7"). He then proceeds to discuss observations he's made of pairs at particular separations - but a separation is a radius, not a diameter.

So, using Tom's numbers, the first (bright) diffraction ring will be 0.85" from the centre of the star, not 1.7" from it. Consequently, pairs at 0.85" (spread plus or minus a bit) will be in the first bright diffraction ring. The second diffraction ring will centre around 1.4" (see below).

Second, he refers to pairs at separations of 1.2" -1.9", uneven by 2+ magnitudes, being so placed that "the primary's first diffraction ring is right on top of the secondary". Nope - can't happen - not with an 8-inch telescope that's operating at full aperture.

To have the first diffraction ring centre somewhere around 1.4"-1.7" requires a 4-inch to 5-inch aperture. And, in any case, a diffraction ring that's so wide it has effects from 1.2" to 1.9" (one ring!) is something I've not seen in a good telescope in reasonable seeing, even with my C9.25 that has a bigger CO factor than a C8.

That leaves the location of the second bright diffraction ring to be considered. Because it's around 1.4" (spread, so plus and minus) from the primary, that fits for Tom's observational experience.

That would suggest a very enhanced second diffraction ring. In an unobstructed system, the second ring will have a maximum brightness around 5.5 magnitudes dimmer (~1/200) than the star producing it. Ring enhancement from a 31% CO will brighten that; and less than perfect optics will brighten it further. Looking at Dick Suiter's table in his Star Testing book, 2nd edition, a 31% CO with 1/6-lambda (whole system optics) - about as good as it usually gets- effectively slightly more than doubles the amount of light in the rings, so if we assume roughly proportional brightening of 1st and second rings, that would produce a 2nd ring maybe near 4.5 magnitudes dimmer than the star.

A secondary star in such a ring should however be visible as a brightening in the ring, if seeing is good, collimation is good etc etc, and optics as prescribed, assuming it's around 2 to 3 magnitudes dimmer than the primary - it also loses extra light from the disc into its rings.

Quite a few different scenarios can be constructed here, as we vary this factor or that. And consider interference effects of a star image on another star's diffraction ring. But I think the overall picture is that
(a) it can't be the first diffraction ring at ~1.5" hiding the secondary star with an 8-inch aperture
and (b) the second diffraction ring seems pretty bright, from the description.

Anyone someone want to do some more exact estimates of the least possible brightness of the second ring....? And run some varied-factors scenarios?


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6058508 - 09/01/13 11:50 PM

I suggest you try that and continue exploring the topic.

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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6059631 - 09/02/13 04:36 PM

Quote:

...In an unobstructed system, the second ring will have a maximum brightness around 5.5 magnitudes dimmer (~1/200) than the star producing it. Ring enhancement from a 31% CO will brighten that...



I am not sure about this but it seems that while CO makes the first ring brighter it makes the second ring fainter and surprisignly the third ring is then actually brighter than the second ...
I also do not fully understand what it visually means that the diffraction rings have specific delta_m's compared to the central disk - does it mean that a secondary with the same delta_m simply disappears in the ring (making it maybe a tad thicker in its position) or is such a secondary to see without troubles as the brightness of the diffraction ring is distributed in the ring.
Will certainly have a look myself at this question - but don't have so far a good idea for a quick size change CO maks construction for my refractor.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6059946 - 09/02/13 08:26 PM

Wilfried, thanks for the reminder - from what I've now looked at, the change in intensity of particular rings depends on the ratio of obstruction. So I'll need to rework the numbers, according to various CO percentages, starting with the CO for a C8. There's a simple version of the relative changes in Suiter (2nd edition, p.165) - 25% obstruction enhances the first and third rings relative to unobstructed, 50% obstruction enhances the rings in order 1, 2, 3 again (linear progression). I'll try to calculate some (rough) numbers.

This won't change some of the points I've made earlier. The first diffraction ring can't intrude on a C8 at the separations quoted; and something (what?) affects the visibility of fainter secondary stars in a particular zone that approximates to the second diffraction ring. Puzzles continue.


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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6060435 - 09/03/13 03:11 AM Attachment (17 downloads)

There is a table from Lord's paper on resolving unequal binaries with values for different CO - maybe of interest.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6060656 - 09/03/13 09:05 AM

Thanks, Wilfried. A useful table, as it shows the changing amount of light transferred into the 2nd and 3rd rings, according to changes in CO size. The first ring keeps growing in intensity as the CO increases, but the relative amount of light in rings two and three is changeable - at 0.2 CO and 0.33 CO the 3rd ring is brighter than the second; at 0.4 CO and zero CO the 2nd is brighter than the third.

I'll look further at how this can affect unequal doubles. But it does indicate that your experiments with a diaphragm on your SCT, having the effect of increasing the CO as you stop down, brings in variations in the diffraction rings' appearance and relative brightness, so it can affect the visibility of faint companions in particular positions from to the primary. And, as your aperture changes as the CO is changed, the position of each ring will vary, as well as its intensity. That increases the difficulty of isolating individual factors with diaphragm use. Your new thought, putting various sizes of CO via masks on the refractor, has the advantage of keeping aperture constant.


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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6061106 - 09/03/13 02:01 PM

Fred, might the deltaM of the rings tell us something of the deltaM required to detect the companion located very near that ring?

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Re: How good are reflectors for resolving binaries? new [Re: Asbytec]
      #6061746 - 09/03/13 09:53 PM

Yes, it should - the delta-m of the rings, included in the Lord table, should suggest the degree of interference (bad pun) with visibility of the secondary star.... if delta-m is greater for the star than for the ring, the star will be lost in the ring, and if it's close to the same delta-m for star and ring it'd be very difficult to be sure that a slight ring brightening is a secondary star rather than a seeing artifact. Sidgwick is quite pessimistic on this - using the old term for a minor secondary star (comes), he remarks - "A comes much more than 1 mag fainter than its primary will be imperceptible if its image falls on the first ring...".

An interesting feature of the changing relative brightness of difffaction rings 2 and 3 is that, for a CO of 0.33, the second and third rings are fairly similar, the third ring in this case being slightly brighter. Both, of course, are dimmer than the first ring, but can interfere with visibility of secondary stars where delta-m is large.

I'll write another (not too long ) item on all this later today if I get time; for the moment, interesting features are the change in location (radius) of rings with CO changes, and (from Sidgwick and others) a need to factor in the increased width of the rings with CO.


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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6062032 - 09/04/13 01:31 AM

Fred, exactly what I was driving at. If the first ring is 3 magnitudes fainter, maybe a star less than 3 magnitudes fainter can be seen. Maybe not. I have no idea, but it might be another interesting aspect to explore. It turns out, 42 Ori has a ~3 mag difference. That's interesting.

It may be that the star will have to generate enough contrast to separate itself from the bright ring. Is one magnitude brighter than the ring sufficient, or must is be only 1 magnitude fainter than the primary? For a .33D obstructed, the second and third ring are pretty bright, too, but they are extremely thin. The first and dimmer fourth rings both have some width to them. If memory serves, the second and third rings are actually slightly less intense than an obstructed aperture. (Normalized, I think.)

Above, the third is dimmer than the second. I wonder if that is for an un-aberrant aperture. One may be brighter than the other depending on the amount of spherical aberration present.

This subject always begs the question, is the difficulty due to sheer brightness gradient or does interference patterns change the brightness of a point source in the vicinity of another. Surely the pattern of interference is much more complex than for a single point source. Certainly at the minima of the primary some destructive interference is going on. I wonder if that affects the brightness of a companion star in that vicinity...and /if/ the companion's interference affects the primary, even.

And can a little bit of adverse seeing manipulate both wave fronts so that - lucky snapshot - constructive interference can form an image of the secondary if only for an instant as the primary's first ring fades at that location?


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Re: How good are reflectors for resolving binaries? new [Re: Asbytec]
      #6062068 - 09/04/13 02:39 AM

As already stated - I am not so sure about this (delta_m for rings equivalent to delta_m binary). I have found so far no precise description how this delta_m is calculated - I assume from the relation of the energy distribution. But this would mean the delta_m refers to the whole ring and not a single point of the ring. So the same delta_m would mean that the secondary is easy to detect when sitting on the ring because he is then brighter than the overlapping part of the ring.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: azure1961p]
      #6062128 - 09/04/13 04:46 AM

Quote:

...I'm clouded out here on my three day weekend off or Id be certainly doing doubles these evenings... Pete




Pete, your equipment (200mm Nexton with 35mm CO) would allow some interesting experiments with aperture masks. First you could easily do limit observations regarding resolving doubles (at least a very solid elongation bordering on a rod) by reducing the aperture. Even with 120mm aperture you have still only 0.3 CO. Second you could combine this with enlarging systematically your CO and thus observe how this changes the diffraction pattern and the resolution for a given double up to the simulation of a C8 with 70mm or even larger CO.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6062244 - 09/04/13 07:54 AM

Thanks for the reminder, Wilfried. I had forgotten you addressed that topic.

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Re: How good are reflectors for resolving binaries? new [Re: Asbytec]
      #6063038 - 09/04/13 05:01 PM

To get back to Fred's detailed analysis, which is also supported by the papers cited in my original post, my problems with 1.2 - 1.9" separations are based on observations, and I took the separations from the WDS which usually gets it right. The stars might have closed up a bit, or, more likely, the secondary is much fainter than its WDS magnitude.

I've found the WDS magnitudes to be all over the place. Try observing a few of the Jonckheere pairs. The vast majority, but not all, are 2mv fainter than their published WDS magnitudes.

So. It's quite possible that my inability to make out a secondary at slightly over an arc second is that it's enmeshed in the primary's diffraction pattern. In some stars, this can be quite large. Sirius is now at 7" separation, way over the Airy radius of the telescope, but I can only see the Pup in moments of Pickering 9 seeing, and with an occulting bar stuck in the focal plane of my eyepiece.

Theoretically, the arguments in the optics texts are accurate, but the atmosphere, star brightness, and how well trained your eye is can yield somewhat untheoretical results.


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Re: How good are reflectors for resolving binaries? new [Re: Asbytec]
      #6063385 - 09/04/13 09:02 PM

Norme, you've quite a few matters/questions regarding the diffraction rings. Regarding the visibility of a star on the first bright ring of a primary - how bright it can be relative to the primary and still be seen is difficult to estimate. Sidgwick, who I've quoted above, is pessimistic; Taylor in the Argyle book is somewhat pessimistic.

With regard to 42 Ori, the issue is whether the companion star is on the diffraction ring or in an interspace. That will depend on the aperture of the telescope, which determines where the spaces and rings fall. More on that, and 42 Ori, in a later note.

The relative brightness of rings, as I've indicated, depends on CO. The numbers are different for different CO proportions. Using Lord's data, and comparing 0.33 CO with zero CO: the first diffraction ring is brighter with 0.33 CO, the second diffraction ring is dimmer with 0.33 CO, and the third diffraction ring is similar with each (a tiny bit dimmer with zero CO).

There are other differences - the spacing of rings and interspaces changes with CO. I've used the figures in Lord's table (via Wilfried) to calculate relative spacing of the centres of dark and bright rings in the diffraction patterns, again for zero CO and 0.33 CO.

Interestingly, the percentage increase in location of the centre of each ring (dark or light) relative to the centre of the preceding ring (light or dark) is smaller for 33% CO than for zero CO.

So, for zero CO,
1st dark
1st bright +34%
2nd dark +36%
2nd bright +20%
3rd dark +21%
3rd bright +14%
These are increases relative to the previous ring (dark or light)

EDIT: I've now corrected this table (first percentage increase) from an earlier error.

And, for 0.33 CO
1st dark
1st bright +47%
2nd dark +50%
2nd bright +13%
3rd dark +14%
3rd bright +16%

Edit addition: In some respects, the numbers are misleading; the positions of the 1st bright ring and others beyond that change only slightly; the biggest change is the reduction in the size of the central disc, and the position of the centre of the first dark ring as the Lord table shows. The first dark ring is centred closer to the central point of the diffraction image (1.098 units, compared to 1.220 units for zero CO). Both disc radius and first interspace centre are reduced about 6%, but the centre of the first bright ring is less than 2% closer to the centre of the diffraction image.

I don't at the moment have data for calculating the effective width of diffraction rings for 0.33 CO. I do have numbers for zero CO.

Yes, spherical aberration will play into this. Discussion thus far is for un-aberrated apertures. Real telescopes have deficiencies, small or large, that will have an effect. Feed in your preferred level of SA

"Surely the pattern of interference is much more complex than for a single point source" - I can only agree. But I don't have at present a means to evaluate this in detail.

"Constructive interference" to make stars visible ("for an instant") on the first bright ring? - one can hope... but the images I see through telescopes more often have seeing artifacts that mimic star points.

Edited by fred1871 (09/05/13 03:54 AM)


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6063391 - 09/04/13 09:05 PM

Quote:

As already stated - I am not so sure about this (delta_m for rings equivalent to delta_m binary). I have found so far no precise description how this delta_m is calculated - I assume from the relation of the energy distribution. But this would mean the delta_m refers to the whole ring and not a single point of the ring. So the same delta_m would mean that the secondary is easy to detect when sitting on the ring because he is then brighter than the overlapping part of the ring.
Wilfried




Wilfried, I thought the delta-m of the ring referred to the brightest part of the ring. The rings, like the central disc, have a gradation of brightness across width. Lewis (1914) gives figures for brightness levels across the disc and each ring as well, from which delta-m figures can be easily calculated, at least for the centre of each ring for unobstructed apertures and assumed zero aberrations.


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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6063406 - 09/04/13 09:18 PM

Tom, see my revised details on diffraction rings, their brightness levels, and locations, above.

Re your C8 - the numbers for it will be close to the 0.33 CO.
For that aperture and 0.33 CO, the ring centres are as follows:
1st dark ~0.7" (=Rayleigh)
1st bright 1.0"
2nd dark 1.5"
2nd bright 1.7"
3rd dark 1.95"
3rd bright 2.25"

These numbers are based on the table Wilfried posted here, from Chris Lord's paper.

Interestingly, the 2nd dark ring (the dark interspace between the first and second diffraction rings) occurs in the middle of your "difficult" zone (1.2"-1.9"). If you don't happen to attempt pairs around that separation, but a bit each side of it, you might not see there's a clearer zone in that area.

Alternately, spread of the rings, due to atmospheric haze, stray light on the SCT corrector plate, dust on it, spherical aberration, figuring roughness on mirrors, zones in optical elements, or ??? - enabling light spread - might occur and hide the clear zone. A few years ago I saw an SCT that had nice looking star images but poor resolution of low contrast planetary detail, and checking further (star test) it appeared to have a lot of spherical aberration in the system. This smeared the diffraction ring pattern in focus as well.

Edited by fred1871 (09/05/13 01:42 AM)


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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6063709 - 09/05/13 01:59 AM

Tom, a couple of other matters. We know, following Wilfried's interest in Jonckheere doubles, that older magnitudes for those are highly inconsistent and often wrong. But many of the magnitudes quoted in the WDS are from Tycho, so they should be pretty good. Many of the older magnitudes are also closer to accurate than the typical Jonckheere magnitudes. 'J' doubles are not typical of the photometry presently recorded in the WDS. No doubt the APASS magnitudes for fainter stars will find their way into the WDS as stars are identified.

Sirius - you're not dealing with diffraction patterns here, in trying to see The Pup - rather, it has to do with image spread and flare light. Sirius, as the brightest star in the night sky and a very uneven double as well, demonstrates these problems better than any other. But it's nothing to do with the diffraction pattern.

"untheoretical results"? - do you mean "theory" in the popular usage sense (speculation) or in the scientific sense (the best description we have that takes all the facts into account etc)?
Currently, we don't have a good theory for describing the visibility of secondary stars in very unequal doubles. We might get close to that at some time, and Wilfried's work-in-progress looks promising, along with some earlier studies that appear to give pointers in the right direction - but given the number of variables involved (including individual eyesight) I suspect any theory, which will need to be consistent with what we know of diffraction, will involve conditionals and approximations.


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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6063773 - 09/05/13 03:51 AM

The effects of CO on the diffraction pattern seems rather complicated. As I have not the means to include diffraction theory in my RoT algorithm I have to keep things simpler. There are so many factors influencing resolving doubles involved that I have for many of them resort to the probability approach.
I have now an idea how to realize different CO sizes for my 140mm refractor (details later) and will first also check the assumption that some CO makes splitting of close doubles easier and that larger CO makes resolving of unequal doubles more difficult. Nest I will also check the effects of CO size on the brightness of the rings and the relation of ring brightness and magnitudes of secondaries. Will take some time.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6064828 - 09/05/13 04:56 PM

Fred,

Thanks for your lucid comments regarding the resolving ability of a telescope on a given double star.

Last night was especially good, and I decided to go for some very close pairs. I'll just list the raw results:

ES 187: 18:23:45.03 +51:38:55 (J2000). Double. Sp: G5
Sep: 2.6, PA: 206, Year: 2008 9.32-9.43

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:13:21
A lovely pair of almost equally bright stars.

HU 940: 19:5:30.85 +33:52:21 (J2000). Double. Sp: F8
Sep: 0.5, PA: 194, Year: 2010 9.18-9.78

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:22:19
A faint oval star, elongated in the WDS pa.

A 260: 18:57:34.06 +32:9:20 (J2000). Double. Sp: A0
Sep: 0.9, PA: 246, Year: 2010 9.17-9.60

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:27:10
Elongated in the expected direction.

A 703: 19:7:12.16 +44:50:30 (J2000). Double. Sp: G0
Sep: 0.6, PA: 189, Year: 2009 9.01-9.28

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:36:49
Not split or elongated.

HU 1300: 19:20:12.77 +34:10:52 (J2000). Double. Sp: A3
Sep: 0.7, PA: 182, Year: 2010 8.92-9.56

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:46:50
Elongated, not split, in the 2.8.

BU 1132: 19:43:14.78 +27:1:6 (J2000). Double. Sp: B8
Sep: 0.6, PA: 214, Year: 2007 9.35-9.94

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:59:35
Barely elongated in the 2.8. It looks single in the 6.


Note that I use 9, 6, and 2.8mm orthoscopics (the 2.8 is the Takahashi, which has a built in barlow) for this work. At under an arc second, I really don't split anything, but I can elongate it. The very high power (725x) of the 2.8 is sometimes (not often) the only way I can get a position angle on a close pair, like BU 1132. As to the telescope itself, I've not had it on an optical bench, but it's bokeh inside and outside of focus is close to identical, better than in my C-11. It has a refractor like "snap" as a star comes into focus. So while I can't give you a strehl ratio for it, it seems to have pretty good optics.

Note that I couldn't even elongate A 703, but if the star magnitudes are accurate, it should have been plain.

If you're still with me after all of that dull text, now I can define "untheoretical". It's when in theory I should easily split a star in good seeing, but I don't. Other "hidden variables" are at work that I don't quite understand.

Bill Hartkopf, of the USNO, confirms that they are trying to get the APASS magnitudes into the WDS. Their budget for this is quite limited, and they are suffering under the sequester, so don't expect anything soon. In the mean time, you never know what you might find when you look at a pair, although Night Assistant does show what other catalogs list about a given star. This helps me know what not to observe.

Getting an accurate algorithm for predicting the resolvability of a given pair with a given telescope is a daunting task at best. Look at the complexity of Brad Schafefer's algorithm for limiting magnitude of a telescope:

http://adsabs.harvard.edu/full/1990PASP..102..212S

I suspect that the one for binary resolvability will be more convoluted. Wilfried is doing us a valuable service.


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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6065307 - 09/05/13 10:11 PM

Tom, your list of observed pairs will no doubt provide some new data for Wilfried, as some of these appear to be near-enough to being "limit" observations. That's useful.

Your comments about your telescope and observing details are of interest. Commercial SCTs do vary a fair bit in optical quality, but some are surprisingly good. I've used a number of C8s in the past, and looked through others; the best one had very good optics, immaculate star images, essentially zero image shift in focusing, etc. It had refractor-like focus snap too.

I'm a little surprised you don't get splits below 1 arc second. I've seen splits below that on fairly equal pairs - say at 0.8" - with a couple of C8s, and not only the best one. My C9.25 gave a "discs-kissing/round, but just about touching", effect on Zeta Bootis recently; that's at Dawes for the aperture (0.5"), and a good C8 should do similar things at around 0.6", unless the stars are too faint to see well. Try mag 6 or 7 pairs, and see what happens.

Orthoscopic eyepieces can be very good for doubles, especially with uneven pairs, because they can have very low light scatter. I saw a striking version of this with my 140mm refractor, when observing Saturn - a 5mm Ortho showed faint moons clearly, very close to the planet, that were hard or impossible to make out with a 5mm Nagler T6. The Nagler is a very good eyepiece, but has noticeably more scatter on bright objects. I wouldn't use it for trying to see The Pup near Sirius.

I see a rather big gap between your two higher-power eyepieces. 6mm at 333x; 2.8mm at 725x. Seeing will often limit the ability to use 725x; I'd suggest getting an eyepiece in between, that might do pretty nearly everything 725x can do, and do it with a bit less power, and keeping visibility of dim stars easier. 725x could be overkill for an 8-inch a lot of the time. Perhaps a 4mm Ortho, to give 500x? It's 50% more power than the 6mm, and could be usable more often, and might make fainter stars easier to see well.

And I did some follow-up on one of your doubles. A703, which you couldn't resolve, may as you suggest have wrong photometry. Here I'm suspecting that it's simply a copying error in WDS.

I had a look at the 1996 WDS to see what mags were given before Tycho mags were imported. That gave 8.3 and 10.6, where the new listing is almost even. Visual observers in the early 20th century would notice that large difference between the stars, whether or not their assigned mags were accurate. The combined brightness (seen as a single star, unresolved) would be similar, whether the mags were the new version or the old, so that doesn't help.

Next I went to Hipparcos. Useful. It lists a double, separation 0.57", primary magnitude around 8.4 (Hipp 8.34; Tycho 8.44) and a delta-m of 1.99 (let's call it 2.0). That fits well with the older data too.
So it looks as if the current listing has wrong magnitudes copied in - the old (1996) listing is closer to accurate. On Hipparcos/Tycho, 8.4 and 10.4 would be a fair call.

That would also explain why you couldn't see elongation - it's not only still very close, but the big magnitude difference will hide any elongation effect from the dim secondary. Otherwise, you might have had a result similar to BU 1132 with delta-m of 0.6.


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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6065541 - 09/06/13 03:02 AM

Thanks Tom, will go through your list and give you feedback with the calculated RoT value for your observations.
Appreciate Schaefers work very much and used his approach for estimating the influence of light pollution on TML for the TML check in my RoT algorithm. Schaefers approach is a fully analytical one in contrary to my statistical one. His algorithm uses a row of multiplication of factors and I use separated modules for the factors and add or substract then the values of the results of the modules and hope so to reduce the range of errors. Interesting result of Schaefers algorithm is that the comparison with about 350 TML observations gives a rather large standard deviation and I have therefore concluded that the simple formula 2.7+5*LOG10(D_mm) is good enough for a statistical approach with a rather similar standard deviation.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6066239 - 09/06/13 02:11 PM

Fred,

Again, thanks for the concise, detailed analysis.

I had another good night (rare for around here [near Washington DC], where we often go for a month or so without clear skies) and learned of a few more complications.

Again I went for close pairs. Again, I had a very difficult time with them. I'd go from my 25mm finding eyepiece directly to the 2.8 or 6. I could elongate a pair like HO_457:

HO 457: 20:24:21.08 +29:22:55 (J2000). Sp: A0
Sep: 2.0", PA: 60 degrees, Year: 2008. 9.08-9.18mv.

but I couldn't split it. A pair of 9mv stars (substantiated by the UCAC4, which lists them as 8.7mv) should have been easy. When I switched to the 12.5, there they were! Two ~1.5" blobs 2" apart, looking like a tiny shimmering peanut.

Upon reflection, I think what's going on is that with the higher powers, the diffraction pattern is, of course, being magnified. It no longer looks like a point source, and they appear like wiggling blobs (seeing was ~Pickering 4) of extended light. A tiny blob with an integrated 8.7mv requires a bit of averted vision to make out. Let's see now. Using the 6, with my -6 diopter myopic eye, that works out to about 350x. This means the pair looks like a pair of 9 arc minute blobs 12 arc minutes apart. The resolution of the human eye is at best one arc minute, and once you start using averted vision, it drops precipitously. High powers on faint stars cause me to use averted vision, and thus I see blobs. When I look directly at it, what I see are dancing speckles, elongated in the correct pa. At the lower power, the stars are essentially points, only 4 arc minutes apart, and the light is more concentrated by a factor of 4. No more averted vision is needed, and Viola! I see the pair.

The vagaries our vision, and the atmosphere are going to make an accurate algorithm for binary splitability fairly complex, with terms for seeing and eyesight acuity (both rather subjective) being part of them.

------------------------------------------------------------------------------

Wilfried,

I see you're collecting visual observations, so here's a URL that has all of mine:

http://mainsequence.org/html/obs/Double_Stars.html

Bear in mind as you use these that Burnham (or was it Aitken?) said the first thousand or so observations by an observer weren't to be trusted.


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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6066784 - 09/06/13 08:18 PM

Tom, I get better seeing a lot of the time than you appear to have; though the last couple of years I've had far too often cloudy nights.

But Pickering 5 is about where I start observing double stars. Below that, maybe some wide ones at low power; but they don't look good at Pickering 4, and tight ones are too mushy/difficult.

Even in good seeing, it's possible to over-magnify - resulting in images that don't help seeing what's there. Your description of "blobs" can be a result of too much power as well as poor seeing conditions.

Your example, HO 457, is informative - highest powers, elongated (presumably, smeared images); less power (160x?), two stars, separated. I've had similar experience - one of the fainter pairs I've seen as two stars, not just elongated, is STF 1620 in Virgo - listed mags 9.1 and 10.35, at 2.2" - and seen as two stars at 160x with 140mm.

I tend to start with moderate magnifications, then work up as needed. I find a lot of fairly tight doubles are split at 160x with the 140mm refractor; likewise the C9.25 at 180x. The tighter and more uneven ones can benefit from higher powers, but often enough I find pairs visible at 160x-230x that are harder to see at 285x and much harder at 400x - the extra power is counter-productive.

And that holds for a lot of fainter pairs - there's a balance point, between where the eye has enough separation to see, and where the light gets spread so much that stars are too dim. As Schaefer's work showed, you get benefits going from low to medium power in seeing fainter; but although his graphs plateau at a certain point, meaning no further benefit, I find in the case of close doubles that there's some loss rather than a plateau. A bit less power can make some pairs easier to see.


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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6067529 - 09/07/13 10:23 AM

Tom, assuming 0.34 CO and no light pollution the RoT algorithm gives the following results:

Quote:

...

ES 187: 18:23:45.03 +51:38:55 (J2000). Double. Sp: G5
Sep: 2.6, PA: 206, Year: 2008 9.32-9.43

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:13:21
A lovely pair of almost equally bright stars.

-> Proposed Aperture 77mm - no limit observation, just warming up

HU 940: 19:5:30.85 +33:52:21 (J2000). Double. Sp: F8
Sep: 0.5, PA: 194, Year: 2010 9.18-9.78

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:22:19
A faint oval star, elongated in the WDS pa.

-> Proposed Aperture 263mm - 200mm would be outside the double standard deviation. Was the elongation clear enough to count this as valid limit observation for resolving this double?


A 260: 18:57:34.06 +32:9:20 (J2000). Double. Sp: A0
Sep: 0.9, PA: 246, Year: 2010 9.17-9.60

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:27:10
Elongated in the expected direction.

-> Proposed Aperture 158mm - should have been no elongation but a clear split or at least a rod with magnification x200 or higher


A 703: 19:7:12.16 +44:50:30 (J2000). Double. Sp: G0
Sep: 0.6, PA: 189, Year: 2009 9.01-9.28

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:36:49
Not split or elongated.

-> Proposed Aperture 214mm - should have been not plain but at least possible with 200mm if the advertised data would be correct

HU 1300: 19:20:12.77 +34:10:52 (J2000). Double. Sp: A3
Sep: 0.7, PA: 182, Year: 2010 8.92-9.56

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:46:50
Elongated, not split, in the 2.8.

-> Proposed Aperture 194mm - 200mm aperture should show at least a rod

BU 1132: 19:43:14.78 +27:1:6 (J2000). Double. Sp: B8
Sep: 0.6, PA: 214, Year: 2007 9.35-9.94

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:59:35
Barely elongated in the 2.8. It looks single in the 6.

-> Proposed Aperture 228mm - 200mm are on the lower side of the standard deviation means still a fair chance with very good seeing

...




The high magnifications you use puzzle me a bit because in my experience the image quality gets drastic worse with magnification more than double the aperture in mm - but in my locations seeing is rarely better than Pickering ~7 and most times worse.
As all observations have been for rather equal bright stars only the first component of my RoT model is active meaning Dawes modified with CO for decreased size of the Airy disk. I wonder if 0.34 CO is already too high for any benefit from this effect. This could be an explanation why you get elongations instead of rods or even splits.
Thanks for the link to your observations - impressive. Problem for me is how to find obervations on the aperture limit.
Regarding HO457 - here the proposed aperture is 77mm, so why you could not resolve it with the C8 remains unclear.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6067682 - 09/07/13 11:53 AM

I'm a deep sky observer, but enjoy the challenge of double stars. I've seen the companion to Sirius many times in a 20 inch Dob with Zambuto mirror, including this morning for the first time this fall. As to aperture masks, when I have used them on large Dobs, I've used off axis aperture masks which eliminate central obstruction (at least I thought they did.) I've read Roland's comments about the disadvantage reflectors typically have compared to refractor objectives with regard to more light scattering on mirrors from dust. I think that's true. 20 inches of dust scatters more light than 6 inches of dust, and most Dob mirrors are probably dirtier than most refractor objectives. Big Dobs collect more scattered light, and for lack of a better term, more "scintillated light" when seeing is less than optimal.

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Re: How good are reflectors for resolving binaries? new [Re: kcolter]
      #6068322 - 09/07/13 06:28 PM

Fred,

Congratulations on your excellent seeing. Visiting Australia in your winter and seeing such things as M22, M8, 47 Tuc, or Omega Cen near the zenith in a good telescope is on my "bucket list". One of these months, after I retire!

A note on the higher powers. There's not a single guide that recommends powers higher than 50x per inch, so to increase this by ~50% should be madness. William Herschel, on the other hand reported using 1,000x on his 6" speculum mirror telescope (with a single element lens, mind you!), so I thought I'd give it a try. After having used it for a while, it's too much power for the vast majority of cases. Given the radius of the Airy disk (Center of disk to first minimum) is about 0.6 arc seconds, stars will appear to be 87" across when viewed from perfect skies with my 2.8mm. Let the skies do their usual wiggling, and the C-8 not be in total thermal equilibrium with the outside air, and the disk only expands. Thus my use of the term "blobs" to describe a star's appearance, especially with averted vision.

Quote:

A bit less power can make some pairs easier to see.




This is my observing experience as well. There are few doubles that I think are "best in the 2.8.". Occasionally, there are. I still haven't figured out why this is so.

Wilfried,

According to Celestron, the CO on my C-8 is 0.31. I don't know if this makes much of a difference in your calculations.

I've noticed over the years that the published magnitudes of a star in the WDS are often inaccurate. Just last night I saw my first "L(ewis)" star, L 36:

L 36: 21:45:57.78 +25:32:26 (J2000). Double. Sp: U
Sep: 4.6, PA: 294, Year: 2010 9.5-10.0

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 7, Time: 0:15:36
An "L" star that exists! Mirabile dictu! It's 2mv fainter than its WDS mv. WDS
data: 9.5 - 10.0mv, U, 294° pa, 4.6", 2010.

It's 2mv off. The stars I've listed, that you kindly did your calculations for (thanks!) might well be subject to the same inaccuracies, thereby throwing them off.

KColter:

In my experience, mirrors are not more dirty than lenses, but lenses have one advantage. From the point of view of an observer, all dust is seen in silhouette, whereas dust on a mirror is reflective, scattering light to unwanted places.


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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6068577 - 09/07/13 09:30 PM

Tom, seeing here varies a great deal, as anywhere. Some areas of Australia are as bad as you say your locale is. Where I am has some very good seeing nights, but mostly they're middling, as expected. I get fairly dark skies for a populated area because I'm near the northern edge of the city, and as well a lot of the streetlights are downward-reflector style so there's less stray light. Average seeing where I am is slightly less good than I experienced in 6 months in San Diego, Ca, some years ago.

From your comments: There's not a single guide that recommends powers higher than 50x per inch
For general observing, that might be the case. The better guides point out that close double stars are often viewed with higher powers. And the double star specialists go further.

Couteau spells it out (others do too) : that the "resolving magnification" (which makes the size of the first ring equal to the limit of visual acuity, supposed to be 1 arc-minute) is equal to the radius of the aperture in millimetres. However - to see the diffraction image in detail (as in observing a close binary) "one must use a higher magnification, called the useful magnification, which is usually three to four times and may be up to five times the resolving magnification".

The numbers? - for a C8 the resolving magnification is 100x, but the useful magnification is in the 300x to 500x range. For a 140mm refractor (mine and Wilfried's), the useful magnification is 210x to 350x. In each case, up to a bit over 60x per inch.

Although Couteau suggests that higher than five times the resolving magnification results in "the edges of the image [becoming] badly defined and the eye cannot transmit information", this I'm inclined to think is more often true with larger telescopes where seeing conditions are setting the limit; and Couteau worked mainly with 20-inch and 30-inch refractors.

Various 19th century observers who used smaller telescopes were inclined to use higher powers than 60x per inch for the closest pairs. But I will say that I find 400x on a 140mm refractor (5.5-inch) the useful limit generally - at 570x I find I'm confirming what I saw at 400x, not getting new information. And most of the closest doubles are seen with 285x or 333x.

Christopher Taylor is similar - his use of 820x on 12.5-inches is ~65x per inch.

William Herschel was much inclined to over-magnify. I've not studied whether this was necessary or merely a way of exhausting the possibilities in his case.

You shouldn't be too surprised at finding a Lewis pair that exists. The problem as usual is likely to be photometry, where the WDS might still have very old eye-estimates of magnitude, because newer photometry has not yet been incorporated. Lewis used the Greenwich 28-inch refractor so I expect that stars that are rather dim in smallish scopes (such as a C8) were fairly bright in the 28-inch. It's about a 2 1/2 magnitude gain. That's similar to the gain in going from a 3-inch telescope to a C11. Noticeable.

Different sources of photometry will have different accuracy levels and different degrees of consistency. Jonckheere is one of the worst, and there are a lot of his doubles. Lewis didn't discover many; mainly re-measuring for changes, and orbit calculations.

L 36 - shows quite nicely on DSS plates; and appears probably dimmer than listed, as you say. ~2 mags in visual could be correct.

Scatter in mirrors versus lenses - mirrors tend to higher scatter than lenses even when both are clean. There's discussion of this in various works on optics; and it turns up in the discussions, here on CN, of mirror versus prism diagonals as well.


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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6068746 - 09/08/13 12:14 AM

Tom,

The 50x per inch is a good rule but its not gospel. Part of the reason that rule came about was as a reflexive counter claim to manufactures boasting absurdly high magnification capabilities on instruments of exceedingly small size and just as often poorly fabricated. The fifty power per inch is a fine generalization though - so long as exceptions are observed...

Diffraction pattern study is not the same as say, lunar or planetary observation where contrasts in an extended image must be preserved and often no higher a power value than 40x per inch and more commonly - as you know - 25x per inch. Here the 50x rule is even a little liberal and optimistic. Stars however having no such extended nature we can hope to see, leave us with the effects and artifacts of the instrument itself as representations of the stars through their resultant diffraction patterns produced. These patterns haven't the needs of contrasts that might benefit the Great Red Spot for example. Its a different kind of observing and one a little more specialized than many guidebooks and such would contemplate.

Using 80x to 100x per inch is a relatively tall order still however and a lot is dependent on the seeing as you'd guess but also the robustness of the stellar diffraction pattern to shine through at these unusually small exit pupils. In practice something like 60x to 70x per inch might be a more useful rule of thumb with averegish conditions. Too, small refractors can hit the higher power per inch envelope sooner than medium apertures and certainly large or huge scopes. With my 8" and good seeing under. 600x is fine and simply easier on the eyes when it gets down to the finer details. Stretching it to 800x has yet to be done successfully but I can't blame the scope as atmospheric refraction has been the culprit even on very good nights. It might literally need 10/10 seeing.

Other instances where 100x per inch (or more)!is beneficial is in small high surface brightness planetaries. Stephen James Omeara using 900x with a 5" refractor is a case in point. Too, extremely small discs like the Galilean moons can show details better in medium to larger telescopes when the magnification is as high as 70x per inch. It'd be grossly excessive on Jupiter but Ganymede at a mere 1.7" benefits nicely - of course when the seeing obliges. Ideally contrast is better at say 25x per inch but that doesn't apply here in application because the ability for the eye to discern such incredibly small pale shadings on a virtual dot is too difficult and so the penalty of higher magnification can be put aside here for the sake of creating a large enough image so it can subtend enough area to begin to reveal itself at all.


Pete

Edited by azure1961p (09/08/13 12:28 AM)


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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6068927 - 09/08/13 05:25 AM

Quote:

...Wilfried,
According to Celestron, the CO on my C-8 is 0.31. I don't know if this makes much of a difference in your calculations...



Difference is only a few mm in proposed aperture as all examples are rather equal bright and therefore the influence of CO is limited to the change in size of the Airy disk.
But I assume 0.31 refers to the size of the secondary mirror and there are some additional mm for the mounting.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: azure1961p]
      #6068935 - 09/08/13 05:47 AM

Quote:

The 50x per inch is a good rule but its not gospel. Part of the reason that rule came about was as a reflexive counter claim to manufactures boasting absurdly high magnification capabilities on instruments of exceedingly small size and just as often poorly fabricated. The fifty power per inch is a fine generalization though - so long as exceptions are observed...

Diffraction pattern study is not the same as say, lunar or planetary observation where contrasts in an extended image must be preserved and often no higher a power value than 40x per inch and more commonly - as you know - 25x per inch. Here the 50x rule is even a little liberal and optimistic. Stars however having no such extended nature we can hope to see, leave us with the effects and artifacts of the instrument itself as representations of the stars through their resultant diffraction patterns produced. These patterns haven't the needs of contrasts that might benefit the Great Red Spot for example.



Yeah ... and (providing the seeing cooperates) as much power as is available is probably going to be helpful when trying to measure double stars using a bifilar micrometer.

Max useful power also depends on the acuity of the eye, my old eyes are starting to drift a bit now but when I was in my early 20s I found that planetary detail was better seen at 20D than 25D. Even on tiny discs. 40 years later I need around 30D & have more issues with low contrast details as a consequence ... Not that this has any relevance to double star work, or using the diffraction pattern to collimate a scope under steady seeing conditions. I reckon 80D - 100D is a reasonable "limit" (assuming the optics are impeccable as well as the seeing - and providing you have a tracking mount!)

Quote:

Other instances where 100x per inch (or more)!is beneficial is in small high surface brightness planetaries. Stephen James Omeara using 900x with a 5" refractor is a case in point. Too, extremely small discs like the Galilean moons can show details better in medium to larger telescopes when the magnification is as high as 70x per inch.



Experiences obviously differ: I've found with planetary nebulae the best magnification is never more than 25D: as for the Galilean satellites, I've never seen clear surface detail but what I've suspected has been low contrast & going over 25D - 30D kills it.

Sidgwick suggests as a practical upper limit to magnification, 140xsqrt(D) - which, as it happens, is approximately 50D for an 8 inch aperture, rather more than that for smaller scopes and rather less for larger. (70D for 4", 35D for 16") Given that seeing doesn't improve with aperture, this sort of makes sense. However, if you want to use a power much over 20D, your eyes are going to need to be pretty free of floaters.


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Re: How good are reflectors for resolving binaries? new [Re: brianb11213]
      #6069143 - 09/08/13 09:42 AM

Brian,

Binoviewers have an uncanny efficiency that a lot of folks appreciate. A common comment from observers with binos is they can see the same details as mono but at lower magnifications. You didn't mention binos so I thought Id throw it out there. Right now my binos are too too high powered due to my ocular choices as its still new to me. Its is a nice things when both eyes see the same object - there's this plainness of definition thsts truly refreshing. So to that end you may find you can enjoy the lower power efficiency once again.

Pete

Edited by azure1961p (09/08/13 09:44 AM)


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Re: How good are reflectors for resolving binaries? new [Re: azure1961p]
      #6069178 - 09/08/13 10:02 AM

Quote:

Binoviewers have an uncanny efficiency that a lot of folks appreciate. A common comment from observers with binos is they can see the same details as mono but at lower magnifications.



Again, other people's experience may differ from yours.

I tried a binoviewer & apart from the obvious reduction in "floaters" I had a couple of issues when using them - eye strain & a loss of light transmission which was roughly equivalent to a whole magnitude. I never saw any increase in ability to resolve fine detail or low contrast detail when using a BV at any magnification. Proper binoculars are OKish (though there can still be some eye strain) but I am far happier using one eye per objective.


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Mask for testing effects of CO new [Re: brianb11213]
      #6070975 - 09/09/13 10:24 AM Attachment (5 downloads)

First prototype is ready consisting of a foam ring over the dew cap and an aluminium bar with a thin tube in the center to accept CO plug-in's. The metal part still needs black painting to avoid potential reflections

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Re: Mask for testing effects of CO new [Re: WRAK]
      #6070978 - 09/09/13 10:28 AM Attachment (7 downloads)

The CO plug-in is simply a black disk on a stick to be applied on the tube in the center. I will make such plug-in's in sizes from 0.1 to 0.35 (or larger if needed) for analysis of the influence of different CO values on diffraction pattern and resolution of binaries

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Re: Mask for testing effects of CO new [Re: WRAK]
      #6070982 - 09/09/13 10:31 AM Attachment (7 downloads)

Finally the applied 0.35 CO on the 140mm scope. At least at daylight there is no difference to see between zero CO and 0.35 CO. Will take some time to get reliable results but this setup seems good enough to study the effects of CO for resolving doubles.
Wilfried


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Re: Mask for testing effects of CO new [Re: WRAK]
      #6071547 - 09/09/13 03:24 PM

Did a lot of similar experiments in the 80's. Interesting to learn how your experiment evolves. Applying a CO to a refractor is different from having a scope with a very different design where the CO is actually part of the optical design. Keep us posted!

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Re: Mask for testing effects of CO new [Re: Erik Bakker]
      #6071708 - 09/09/13 05:11 PM

Size of the secondary in a C-8 Ultima.

Today I tried to measure the diameter of my secondary and and its mount in the corrector plate of my C-8. I got 69mm. Plus or minus a mm, at best. This gives, assuming the corrector is 203.2mm in diameter (I didn't measure that), a 34% obstruction.

Wilfried, so you, and not Celestron, were closer to right.


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First experiments with CO mask new [Re: 3c_273]
      #6073209 - 09/10/13 01:15 PM

Last night conditions (cloudy sky with clear spots inbetween) did not allow a regular session so I had time at hand to give my setup a first try.
Polaris was my first target to check the effects on the diffraction pattern for a single star (neglecting the faint companion). Seeing was with Pickering ~5 rather mediocre so the diffraction pattern was not stable but the obvious expected effect of enhancing the brightness of the first diffraction ring was evident especially for 0.3 and 0.35.

Next target was Epsilon Lyr with x75 magnification. Zero CO showed nice dented rods, 0.1 CO made the image a tad more crisp with hints of dark space between the star disks. Not much change with 0.15 and 0.2. 0.25 CO delivered the same impression as zero CO, 0.3 CO deleted the dents in the rods and 0,35 made the rods a tad fat like elongations.

Same target but with x280 magnification: Zero CO showed clear separated star disks with a rather faint first diffraction ring and 0.1 CO again made the image appear a tad crisper. 0.15 and 0.2 did not change much and 0.25 was similar to zero CO with first diffraction ring a bit brighter. 0.3 CO made the image less crisp and the first diffraction rings got bright enough to disturb the dark space between the star disks. 0.35 CO even more of this effect to a degree of making the image more impressive in terms of diffraction rings but less precise in terms of resolution.

Regarding TML CO 0.1-0.25 did not show any obvious influence (on some very faint stars in between the double-double, at least one with +12.676mag according to AAVSO) but 0.3 and 0.35 CO did.
This are only first findings to be confirmed but it seems that the positive influence of CO regarding size are restricted so smaller CO values less than 0.25 and degradation of resolution begins with ~0.3 CO.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6074289 - 09/11/13 01:24 AM

Wilfried, your observations are interesting. My experience with a ~30% co (only) seems to suggest a push from 0.77" arc to closer to 0.70" arc Dawes split. I am still waiting to see if that's actually the case. Need a good target and a good night.

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Re: First experiments with CO mask new [Re: Asbytec]
      #6074377 - 09/11/13 03:59 AM

I'm curious how the indent or notch of a Dawes split would "delete" when the CO became large as the notch in theory should have deepened as the spurious discs appeared smaller due to light going into the rings. I would wonder if the CO mask was now obstructing the zone of good or better definition in the center of the objective elements .

Pete


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Re: First experiments with CO mask new [Re: azure1961p]
      #6074401 - 09/11/13 04:44 AM

Quote:

I'm curious how the indent or notch of a Dawes split would "delete" when the CO became large ...



I think that at low power the first diffraction ring cannot be seen but only the central disks - therefore notched. With increased CO the first diffraction ring gets brighter and "fill in" the gap therefore eliminating the notches. Further experiments will show if this assumption could be right.
Wilfried


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Re: First experiments with CO mask new [Re: Asbytec]
      #6074404 - 09/11/13 04:48 AM

Quote:

Wilfried, your observations are interesting. My experience with a ~30% co (only) seems to suggest a push from 0.77" arc to closer to 0.70" arc Dawes split...




This would be correct from the theoretical point of view and can still be true in reality as the brightening of the first diffraction ring may be without side effects for close doubles. To check the effects of CO on close doubles I need a target near or slightly below Dawes - may be next time.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6074475 - 09/11/13 06:59 AM

Did some research on Eps Lyr - the faint stars I have seen between Eps1 and Eps2 are the components E and F (also named SHJ277) but the given magnitudes are with +11.71 and +11.2 not this faint as I found with AAVSO. I would especially doubt the second value.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6074512 - 09/11/13 07:34 AM

Oh, I see, you were at low power describing disappearing notches. I missed that. Yea, I am very curious to see how close a split can be and still call a dark space...at high power. Hopefully soon.

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Re: First experiments with CO mask new [Re: Asbytec]
      #6074563 - 09/11/13 08:30 AM

At 280x however you are but at low power of course and you are claiming lower resolution . I understand how the first diffraction ring can swell into the gap separating it from the spurious disc particularly in 5 Pickering. Still Lords claims a benefit on near equal doubles with a 72% CO. Im guessing the focus at the edge of your objective lens elements is not the same as the masked off center. Perhaps its a refractor thing. Lords did it with his 12 1/2" f/7 reflector - a whopping CO over 9"!!!


Pete


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Re: First experiments with CO mask new [Re: azure1961p]
      #6074583 - 09/11/13 08:45 AM

Pete, I am talking about x75 - x280 was different, see above.
With CO 0.9 you see nearly nothing besides diffraction rings - I know it because I have done it.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6076137 - 09/12/13 12:01 AM Attachment (12 downloads)

I always found this interesting...

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Re: First experiments with CO mask new [Re: Asbytec]
      #6077419 - 09/12/13 05:01 PM

Norme - great graphical representation of the influence of CO on the energy distribution. Matches perfect with what I have seen with CO values >0.4. What is the source?
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6077815 - 09/12/13 09:49 PM

Wilfred, someone attached that to a thread long ago, I grabbed it because it was interesting. It shows the diffraction ring only pattern at 90% you mentioned, and it even shows the decrease in the first minimum.

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Re: First experiments with CO mask new [Re: Asbytec]
      #6077824 - 09/12/13 09:59 PM

Yes it does Norme. That's actually a fantastic graphic.

Ok currently elongating a pair at .021". I'm at 250x of course or it wouldn't be possible.

Pete


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Re: First experiments with CO mask new [Re: azure1961p]
      #6078073 - 09/13/13 12:27 AM

I dunno, Pete. There are so many factors involved. First, it's close or beyond what appears to be an empirical limits at 0.5R. That limit might be pushed depending on conditions and personal ability to note any minor elongation - if it exists in such a tight PSF at all.

That's my question. When in theory do two points merge into a single and indistinguishable PSF? I say define that limit and try to observe to it. Damn the conditions, try it.

Someone with the right conditions, CO, and personal skill might be able to press a few hundredths of an arc second off empirical observations provided that separation does not exceed the hard limit of the indistinguishable PSF. That limit may well lay right at or slightly below 0.5R. It does with extended objects, but does it apply to two point sources, as well?

Edited by Asbytec (09/13/13 12:28 AM)


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Re: First experiments with CO mask new [Re: Asbytec]
      #6078177 - 09/13/13 02:58 AM

All interesting stuff, and there's the reminder that the observer's ability to recognise (see) what's there is the final link in the chain - and why there are different limits even when it's the same telescope on the same night on the same object.

The animated diffraction patterns item, posted above, is useful, but I'd love to see it as a set of diagrams, which would allow studying the effect at each level of CO.

But finally it is, as you say Norme, what level of overlapping diffraction patterns, merged into some kind of image - near 0.5R or some other fixed point - can be seen elongated rather than round by the observer. And which observers can be outstanding in visual acuity, by consistently doing better than other careful and experienced observers, without getting into false positive territory?


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Re: First experiments with CO mask new [Re: fred1871]
      #6078244 - 09/13/13 05:49 AM

I dunno, Fred, how to answer that. One has to respect the experienced and expert observers and take what they say seriously. Anytime we observe hard and deep right at the limits of every variable present we risk false positives, our reputation, and our own integrity. No pain, no gain.

Good seeing here minimizes one of those many variables and offers some hope when cooled and collimated - leaving skill and luck remaining. The best I've done so far is 0.57" arc (72 Peg data as reported.) That turns out to be 0.57/0.92" ~ 0.6R normalized or ~0.7R with CO factored in. It wasn't entirely difficult, and suggests one can go even deeper.

That also implies, with CO, one might elongate Chi Aql in a 6" obstructed depending on it's real sep. That's a real test of 0.5R and CO affects. So far - one attempt in less than ideal conditions - has been negative. But, wouldn't that be a nice ob to have under your belt if it were possible and people believed you? How many look at it's reported data and immediately write it off as too hard or impossible? (That was my point with Io in another thread.)

And 7 Tau seems to suggest there is room to go beyond Dawes, too. All of this provides some allure for doubles and the challenge they pose is motivating. It's amateur frontier observing pushing technology and skill. That's exciting as it is risky.


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Re: First experiments with CO mask new [Re: Asbytec]
      #6078423 - 09/13/13 09:38 AM

Norme, I agree about trying, to see what's possible. I hadn't thought I could see Zeta Bootis elongated, when it was down to 0.5R for my 140mm refractor, until I managed it. And I have no trouble believing some observers see hints of elongation, or something? ... at even closer (slightly closer) levels. Christopher Taylor's claimed best is ~0.4R with his 12.5-inch. Aitken's note on detecting doubles with the 12-inch at Lick goes down to ~0.45R...

The two doubles you mention after your success with 72 Peg are somewhat different types of critters - Chi Aquilae is at 0.5R for about a 7-inch scope based on separation (~0.4") - although delta-m of 1.2 will have some effect.

But 7 Tauri is wider, and more equal - mags 6.60 and 6.86 in the current WDS, 0.7" in 2012, and the ephemeris (gd 3 orbit) suggests 0.75" at present - so that's a 0.5R for about a 4-inch scope. I observed it last November, 140mm refractor, in rather unsteady seeing - elongation was visible at 285x, with star discs (peanut or notched effect) flickering in and out of visibility. That's at about 0.7R for 140mm. Some other doubles at about the same separation were clear flattened figure-8 pairs on nights with better seeing. At 0.6" - which is around 0.6R at 140mm, having a Rayleigh figure ~1.0" - I find elongation is clear enough if the seeing permits. It's consistently there at 400x; sometimes a bit less power will show elongation in the best conditions.

But below 0.6R things get noticeably tougher, and more demanding of steady atmosphere for allowing the certain identification of elongations, rather than uncertainty due to seeing variations. For me, 400x becomes necessary for definite identifications of elongation at this level.

Personally, I find it more interesting to try for the harder unequal pairs; but I'm happy to see others working on their limits for detectability of equal pairs. It's an interesting pursuit.


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Re: First experiments with CO mask new [Re: fred1871]
      #6078536 - 09/13/13 11:10 AM

Quote:

...
The animated diffraction patterns item, posted above, is useful, but I'd love to see it as a set of diagrams, which would allow studying the effect at each level of CO. ...



Fred, with the old fashioned but free Microsoft GIF Animator you can see the single planes in the preview mode - or you could cut out each frame and save it as non animated image.
Wilfried


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Re: First experiments with CO mask new [Re: fred1871]
      #6079544 - 09/13/13 10:35 PM

Quote:

Personally, I find it more interesting to try for the harder unequal pairs...



Absolutely, I love the challenge they pose as well. They are interesting from that standpoint as well as interesting in optical theory of obstruction effects. This is one reason why I modified my scope, to press these babies, too.

Always a nice write up, Fred. As a newbie to doubles, I find them fascinating on many fronts, especially pushing the limits of man and machine while enjoying nature's beauty.


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Re: First experiments with CO mask new [Re: Asbytec]
      #6079660 - 09/14/13 12:11 AM

For the record I was joking about the 250x showing .21". I didn't see any chortles or chuckles so I'm guessing it was taken without humour .

Pete

Edited by azure1961p (09/14/13 12:14 AM)


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Re: First experiments with CO mask new [Re: azure1961p]
      #6079952 - 09/14/13 08:23 AM

Fred's statement "...Zeta Herculis, as observed with my two scopes - just elongated this year with 140mm..." made me aware that Zeta Her would be a very interesting object for testing different CO values. With a separation of 1.3" (or ~1.2" according the 6th orbit catalog) the companion should sit directly at the first diffraction ring when using a 140mm scope. Thus I could check with which CO value the brightness of the first ring is ident with the brightness of the secondary giving a hint for the magnitude of the ring in terms of a part of the ring comparable to the spurious disk of a star and not for the ring in total.
What surprises me here is the statement "elongated" as this would mean a separation of less than 1".
Its now already a bit late in the season for me to observe Zeta Her in reasonable altitude so I need the next clear sky night rather soon.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6081206 - 09/14/13 10:22 PM

Wilfried, the reference to Zeta Her should have been Zeta Bootis - sorry. Both are doubles, but Zeta Her is easy; Zeta Boo is not at present. Having looked at both recently, along with all the diuscussions of 90 Herculis etc etc, I apparently wrote "Herculis" when I meant "Bootis". Zeta Boo is at 0.5" this year (2013.5); hence just elongated with 140mm, barely split (discs virtually touching) with 235mm.

That should eliminate any puzzle in the matter.

Yes, Zeta Her could be a test object for the secondary on the first diffraction ring if it's at 1.3".


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Re: First experiments with CO mask new [Re: fred1871]
      #6082855 - 09/15/13 09:34 PM

A follow-up on Zeta Herculis - the magnitudes are 2.95 and 5.40 (WDS), separation 1.3" (2011). With a grade 1 (definitive) orbit, of only 34.45 years, the ephemeris should be pretty accurate - it suggests ~1.2" this year. Need for a slight revision to the orbit? - without a full list of measures, especially recent ones that are presumably of higher accuracy due to better techniques, not possible to say.

I looked at Zeta Herculis in July this year with the 140mm refractor - my notes from the night include "at 160x the companion shows as a disruption to the first diffraction ring, occasionally as a spot on the ring...". The varying aspect was due to seeing fluctuations; on the night seeing was generally flickery, and where I'm located zeta Her only rises to about 23 degrees altitude.

So you're quite right Wilfried - it's an interesting one for 140mm (at present) because the secondary is on the first diffraction ring. But it showed quite well despite that and a delta-m of 2.45 - however the brightness of the secondary star I'm inclined to think, helps with bright pairs like this. When the secondary is at similar delta-m, but a lot dimmer - say mag 8 or 9 - it can be more difficult.


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Re: First experiments with CO mask new [Re: fred1871]
      #6099739 - 09/25/13 03:35 AM

After weeks clear sky last night. Started rather early to get Zeta Her with reasonable altitude but seeing was really bad with Pickering rather 4 than 5 - so no resolution with 140mm. Hoped for improvement and continued the session in Her trying 90 Her and 99 Her - as to expect negative. Proceeded then to STF2166 2.3" +8.8/9.08mag - easy resolved with x75, smallest resolving aperture was 70mm. Switched then to the CO mask (means back to 140mm) using x200:
CO 0.1 - no obvious effect on the image
CO 0.15 - disks a tad smaller and crisper
CO 0.2 - no obvious change
CO 0.25 - no obvious change, disks may be a tad smaller
CO 0.3 - first hints of diffraction rings, else no obvious change
CO 0.35 - diffraction rings brighter, disks fainter, impression of less crisp resolution.

Switched then to Aql and had a look at Pi Aql 1.5" +6.34/6.75mag. Clear resolution with x140, limit aperture 65mm giving a notched rod. Switch to CO mask (aperture 140mm, magnification x200):
CO 0 - no rings, clear resolution
CO 0.1 - no change
CO 0.15 - image tad crisper, resolution seems enhanced
CO 0.2 - similar image with first hints of diffraction rings
CO 0.25 - disks smaller, rings brighter
CO 0.3 - disks smaller, rings brighter
CO 0.35 - more of the same, resolution still intact.

Switched then to STF2616 3.3" +6.85/9.64mag means larger delta-m. Resolution with x75 but companion as to expect rather faint, did not check limit aperture, only CO.
CO 0.1 - same image
CO 0.15 - companion tad fainter
CO 0.2 - companion barely visible
CO 0.25 - companion resolution only with moving through field of view
CO 0.3 - diffraction ring of primary gets brighter than companion, resolution only when moving
CO 0.35 - resolution of companion only when moving very difficult and only for fractions of seconds.

Next object A1663 1.3" +8.89/9.25mag - resolution with x140, did not check limit aperture, only CO.
CO 0.1 - same image
CO 0.15 - crisper image, better resolution
CO 0.2 - no visible change
CO 0.25 - slight degradation of image back to CO 0 resp. 0.1
CO 0.3 - no visible change
CO 0.35 - disks smaller but a bit fuzzy, resolution somewhat worse than without CO.

First conclusions:
- Will make an additional CO 0.4 insert to verify my impression that CO 0.35 seems a critical value for image quality
- Small CO 0.1 to 0.25 effects seems conform to diffraction theory meaning enhancing resolution for equal bright binaries to some degree
- CO 0.3 seems to be the break even point for equal bright pairs
- CO 0.35 leads already to some degradation making resolution a tad more difficult but not seriously
- for unequal doubles with delta-m larger 2 CO 0.3 and larger makes resolution more difficult and can get a limiting factor
- bad seeing has certainly influenced this observations so this conclusions are to be taken with caution.

Hope for better seeing next time to continue with this experiment.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6099768 - 09/25/13 04:40 AM

Very impressive report on the effects of increasing CO on doubles. The rewards of an increasing CO are more compactness and, compared to refractors, less color. Currently I have three instruments, with different CO's:

4" f/8 apo CO=0
16" f/5 dob CO=0.17
5" f/11 SCT CO=0.37

I like them all.

The image quality the 5" is capable of, with it's large 0.37 CO, is what continues to surprise me. When the skies clear, I will make a direct comparison between the 4" and the 5". Just had this scope for 2 weeks. Of course, the CO lowers the peak intensity of the Airy disk, while at the same time pushing more light in the brighter rings. That is obviously visible in the 5" SCT.

The same effect is already very difficult to observe in the 16" Newt. Under good conditions, that scope behaves more like a hypothetical 16" apo with 0 CO. Doubles are stunningly pure and clear with perfect chromatic correction in my bino with the 16" on those good nights. That too has surprised me.

Per inch of aperture, the 4" rules. With it's excellent peak intensity and very little energy in the rings, the images are of unmatched clarity. But only per inch of aperture. The overall high correction in this small scope makes itself known in very high contrast and great tranquillity of the images it presents to the observer.

All in all, your findings are very similar to what I've seen at the eyepiece so far. Keep up the food work and let us know what the 0.40 CO does to the image in your scope!


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Re: First experiments with CO mask new [Re: Erik Bakker]
      #6099898 - 09/25/13 08:24 AM

Erik,

I agree the SCT (mine being a 6") will show surprisingly fine doublestar views when seeing cooperates - and - its cooled and well collimated of course. The rings are brighter of course, but the optics are so very good I honestly don't mind. It'd be a bother if a faint companion were interfered here by them but for all others Ive enjoyed some very pleasing views here too. Porrima was stunning!!! Wilfreid nice defines the effects of CO here and on a number of pairs but from a personal standpoint I can forgive a 37% CO when the optical train is top shelf.

Wil, an excellent account and on a revealing variety of doubles . The poor seeing was actually appreciated as its part of the observing condition - particularly where I live during winter.

Pete

Edited by azure1961p (09/25/13 08:34 AM)


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Re: First experiments with CO mask new [Re: azure1961p]
      #6100837 - 09/25/13 04:42 PM

To avoid misunderstandings - I do not intend to make anybody feeling bad about his or her scope with CO 0.35 or larger. I just want to get a better understanding of how CO influences resolution of unequal binaries to enhance my current RoT algorithm showing a weakness here. It also seems that potential negative effects of large CO values are enhanced by not this good seeing and may be neglectable with very good seeing.
Wilfried


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Re: First experiments with CO mask new [Re: azure1961p]
      #6100866 - 09/25/13 04:54 PM

Wilfried, it interesting you mention a peal performance near a 0.3D. As Erik says, the CO of course reduces peak intensity. For an optic of Strehl 0.95, it can maintain a 'diffraction limited' peak intensity of ~.80 when the CO is about 0.3D. This was my motivation to get my CO down from about 37% effective to about 31% real (at full aperture, and since it has such a small FOV, anyway.)

A CO larger than 0.3D, aberration less than 0.95, or both begins to reduce peak intensity below 0.80. It should begin to become noticeable in performance, if we're to believe the Raleigh criterion (anything less than 1/4 P-V SA can be considered diffraction limited - equivalent CO induced diffraction.)

You can approximate the peak intensity using comax~[0.6-(0.6Sn/S)]^1/2, where Sn is the peak intensity desired and S is the Strehl. It turns out in my own scope, Strehl 0.94, to maintain 0.8 peak intensity the CO must be 0.3D. Mine is 0.31D, so just a tad under 0.8 peak intensity (normalized.) The equivalent wavefront error can be estimated using w~0.21co for 0.4D and smaller. For example, a 0.3D is approximately equivalent to 0.063 RMS for the left side of the MTF.

The right side MTF resolution is still 1/(1-co^2) times better - about 10%, even though peak intensity falls below 'diffraction limited' performance. The latter effect is what I suspect you're beginning to notice at 0.3D, peak intensity falls off below the standard 'diffraction limited' performance. This is due to both obstructed light loss and added diffraction contributing more light to the rings.

http://www.telescope-optics.net/obstruction.htm

So, I am not really surprised by your findings. The sound consistent with theory some of which being empirical, too. That you're noticing a drop off in perceived performance, noting the changes in the central disc, is a testament to the idea diffraction limited is indeed minimal performance.

Edit:
Quote:

...make anybody feeling bad about his or her scope with CO 0.35 or larger.



Of course not, CO as large as 0.4D are optimized for performance along the entire MTF as well as providing wider fully illuminated FOV for improved versatility. If anyone wants feel bad, my small and tight fully illuminated FOV is estimated to be about 10' arc.

Edited by Asbytec (09/25/13 05:19 PM)


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Re: First experiments with CO mask new [Re: Asbytec]
      #6101666 - 09/26/13 02:27 AM

Quote:

... This was my motivation to get my CO down from about 37% effective to about 31% real (at full aperture, and since it has such a small FOV, anyway.)...




Interesting ... how did you manage this?
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6101674 - 09/26/13 02:46 AM

Quote:

...how did you manage this?




Cut off the secondary baffle reducing the size from ~53mm and 140mm effective aperture to ~47mm and 150mm full aperture.

So far, no ill effects from stray light. The un-vignetted field is very small with the diagonal and the remaining primary baffle is very tight. A little flocking along the primary baffle through the visual back and stray light is pretty much eliminated.

I was a bit concerned over exposing the image to peak aberration from the primary's edge. Star testing and visual observation show no significant additional aberration.


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Re: First experiments with CO mask new [Re: Asbytec]
      #6102780 - 09/26/13 06:11 PM

Quote:

... For an optic of Strehl 0.95, it can maintain a 'diffraction limited' peak intensity of ~.80 when the CO is about 0.3D ... A CO larger than 0.3D, aberration less than 0.95, or both begins to reduce peak intensity below 0.80. It should begin to become noticeable in performance, if we're to believe the Raleigh criterion ... You can approximate the peak intensity using comax~[0.6-(0.6Sn/S)]^1/2, where Sn is the peak intensity desired and S is the Strehl... http://www.telescope-optics.net/obstruction.htm
...




Norme, thanks for this informations - very useful, especially the formula for Peak Intensity depending on Strehl and CO size and the connex with the Rayleigh criterion. So far theory is conform with my observations and I am waiting for the next clear sky to continue. Currently I am very interested in the effects of CO for doubles with a fainter companion in or very near the first diffraction ring and with which CO value the companion is no longer to resolve - the result would enable an educated guess regarding critical delta_m values of unequal binaries and a better interpretation of the effect of the amount of energy in the first ring of the primary. Several sources indicate a delta_m between central disk and rings but so far I have found no clear explanation how these values are derived and how they are to interprete (for the whole ring or for each point of the ring corresponding to the size of a central disk of a companion).
Wilfried


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Re: First experiments with CO mask new [Re: Erik Bakker]
      #6102818 - 09/26/13 06:30 PM

Quote:

... the 16" Newt. Under good conditions, that scope behaves more like a hypothetical 16" apo with 0 CO. Doubles are stunningly pure and clear with perfect chromatic correction ...




Erik, meanwhile I think Newtons with f/9 or larger focus ratio and small CO may be perfect scopes for resolving doubles as counter intuitively a small amount of CO seems to enhance the image quality for this purpose. Only two arguments against them: Too large for easy handling and demanding regarding seeing.
Your positive report regarding a f/5 Dobson for resolving doubles surprises me somewhat as such a small focus ratio should be combined with reasonable coma.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6103047 - 09/26/13 08:58 PM

Wilfried, yes, that's an interesting problem locating a binary companion on the first ring (or any ring) relative to delta_m. I'd like to know more about that as well.

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Re: First experiments with CO mask new [Re: WRAK]
      #6103266 - 09/26/13 11:33 PM

Wilfried, a useful study, despite the limited number of objects, and the separate variable of seeing, interacting with CO. Not surprising that you note the "bad seeing has certainly influenced this observations" - so we'll look forward to seeing what results you get in better seeing conditions. It's been an accepted principle for a long time that seeing interacts with CO to create less good images, so one would expect with your methodology that inferior seeing more strongly affects your telescope (adversely) when you're using a large CO, compared to no CO. It will be useful to look further at the CO effect, 0.30 vs 0.35 vs 0.40, in better conditions. At 0.40 I'd expect markedly inferior image quality (compared to zero CO) if the seeing is mediocre or poor - and less deterioration in good seeing despite the large CO.

Looking at your results, I'm assuming that "CO 0.2 - no visible change" refers to comparing with 0.15 CO, the previous line, not with zero CO.

One result of particular interest is that 0.15 CO resulted in several cases with "crisper image, better resolution". With those doubles, the magnitudes were nearly equal. A different result with STF 2616, where delta-m is 2.8 magnitudes - there, the 0.15 CO result was "companion tad fainter" and 0.20 CO gave "CO 0.2 - companion barely visible". Quite a different effect, and similar to Christopher Taylor's comments on his observation of a larger-delta-m pair, with 0.17 CO.

Your current impression that "CO 0.3 seems to be the break even point for equal bright pairs" , if it continues to fit your observations, may well be a useful finding for observers wishing to maximise resolution of equal pairs. So observers with, say, a Newtonian that has 0.15 or 0.20 CO might add a central disc to enlarge CO to 0.30 when observing very close equal pairs. With unequal pairs, the smaller CO would do better.

More thoughts on this later.

Erik Bakker's 16" f/5 - I'm wondering if he uses a coma corrector... the diffraction-limited field for an f/5 of that aperture is quite small.

Another issue with Newtonians, apart from coma (short f/ratio) and bulk (long f/ratio) is tube-currents and cool-down. I've seen major problems with some Newts because of those factors, more troublesome than with refractors.


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Re: First experiments with CO mask new [Re: fred1871]
      #6103485 - 09/27/13 03:37 AM

Quote:


Erik Bakker's 16" f/5 - I'm wondering if he uses a coma corrector... the diffraction-limited field for an f/5 of that aperture is quite small.

Another issue with Newtonians, apart from coma (short f/ratio) and bulk (long f/ratio) is tube-currents and cool-down. I've seen major problems with some Newts because of those factors, more troublesome than with refractors.




I don't use a coma corrector. On-axis, images are superb anyway. In my bino at 150x and higher mags, a sizable portion of the field is sharp. The images in the 16" f/5 are so very refractor like. Unlike in my Questar 7, where the first ring was so "large obstruction" like. In the Q7, unequal doubles where hindered by the lowered peak intensity and fat first ring. Not in the 16" f/5.

It's minimalist design works very well thermally too. Ever seen a fast cooling 16" apo refractor
And yes, the 4" apo cools faster. But the Q7 a lot slower.


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Re: First experiments with CO mask new [Re: fred1871]
      #6103522 - 09/27/13 05:38 AM

Quote:

Looking at your results, I'm assuming that "CO 0.2 - no visible change" refers to comparing with 0.15 CO, the previous line, not with zero CO ...




Yes, correct

Quote:

... Your current impression that "CO 0.3 seems to be the break even point for equal bright pairs" , if it continues to fit your observations, may well be a useful finding for observers wishing to maximise resolution of equal pairs. So observers with, say, a Newtonian that has 0.15 or 0.20 CO might add a central disc to enlarge CO to 0.30 when observing very close equal pairs. With unequal pairs, the smaller CO would do better...




Fred, language is tricky, this is a misunderstanding: Break even means crossing the line to the worse compared with zero CO, not turning point of the peak.
My impression so far is that for equal binaries the peak performance (means better than refractor) is between 0.15 and 0.2 CO. With 0.25 CO you have first evidence of a slight degradation compared to the peak but you can still expect refractor like performance with somewhat brighter first diffraction ring and over 0.3 CO troubles begin at least for fainter equal binaries. To some degree this can be compensated with larger aperture but with CO values above 0.4 things get this bad that not even larger aperture can compensate the negative effects - such a scope is then only useful for bright equal pairs and especially for very faint wide pairs (also a very interesting field, you just have to know it).

For unequal binaries the rules change and depending on separation and delta_m already small CO values have a negative impact as a secondary with less peak intensity can get lost in the brighter diffraction rings.

Disclaimer: This are first impressions from a few observations with rather bad seeing and the conclusions may be premature.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6103865 - 09/27/13 10:27 AM

Thanks for the clarification, Wilfried. I got in ahead of the further data collecting. Though I am wondering if a CO of 0.30, or near that, might prove to be useful for equal pairs when the seeing is good - reducing the diffraction discs, but not suffering too much loss of quality in better conditions, and with the enhanced diffraction rings not being too dominant, at least for the brighter doubles. We shall see.

Very large CO - 0.4 and bigger - I've never thought good for visual use. Twenty years ago I got to use a 16-inch RC with large CO - probably a bit above 0.4 - and thought it less good on globular clusters than a C14 I was familiar with.

Yes, "conclusions may be premature".


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Re: First experiments with CO mask new [Re: Erik Bakker]
      #6103898 - 09/27/13 10:41 AM

Quote:

Quote:


Erik Bakker's 16" f/5 - I'm wondering if he uses a coma corrector... the diffraction-limited field for an f/5 of that aperture is quite small.

Another issue with Newtonians, apart from coma (short f/ratio) and bulk (long f/ratio) is tube-currents and cool-down. I've seen major problems with some Newts because of those factors, more troublesome than with refractors.




I don't use a coma corrector. On-axis, images are superb anyway. In my bino at 150x and higher mags, a sizable portion of the field is sharp. The images in the 16" f/5 are so very refractor like. Unlike in my Questar 7, where the first ring was so "large obstruction" like. In the Q7, unequal doubles where hindered by the lowered peak intensity and fat first ring. Not in the 16" f/5.

It's minimalist design works very well thermally too. Ever seen a fast cooling 16" apo refractor
And yes, the 4" apo cools faster. But the Q7 a lot slower.





Useful information, Erik; though I'm inclined to think the large apeture helps make up for the coma limitations for visual use, if you're only looking at the centre of the field. At 150x, observers typically see to about 1" resolution, so the 1" field is much larger than the diffraction-limited field for a 16-inch. Even at 300x that remains true, though the size of the 1/2" field is obviously smaller (though enlarged more for the observer at that power).

A quick calculation suggests diffraction-limited field for 16" f/5 is about 75" (1.25') - not very large. But because coma scales with f-ratio for Newtonians, the 1"-defined field is about 4.5' wide, much more substantial.

So I'm not surprised that, with small CO, the images are refractor-like. A good Newtonian does do that, as I've seen with some Newtonians I've owned, and others I've looked through, over the years. However one of the more impressive short Newtonians I've looked through in recent years was an f/4.5 with coma corrector added, and a superb mirror - very refractor-like indeed.


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Re: First experiments with CO mask new [Re: fred1871]
      #6105167 - 09/28/13 12:42 AM

Quote:

It's been an accepted principle for a long time that seeing interacts with CO to create less good images...



Fred, that's an interesting point, though I am not aware of any sources that speak to that nor what mechanism would cause an image to be worse with a CO. It's reasonable seeing affects the wavefront in the field reducing image quality. If a scope has a high Strehl, it might be more resistant to seeing induced aberration up to a point.

In that case, seeing seems to affect the wavefront pretty much equally in a given aperture despite the presence of the CO which has already done it's damage. A CO adds to diffraction effects but does not affecting the wavefront (other than improving it slightly by masking the center of a spherical optic.) Induced wavefront error is more affected by aperture relative to R0.

However, since obstructed scopes are already close to a nominal operating peak intensity closer to the diffraction limit, it could be that seeing pushes peak intensity below that level quite easily thus "damaging" the image. Still, in smaller apertures where D/R0 ~ 1 (about 8/10 Pickering), the image is still diffraction limited despite the CO.

It's a curious aspect of seeing and I have not run across any studies speaking to the effect of a CO. Most studies discuss he affects on aperture and do not discuss the CO. It's intuitive, however, to understand how peak intensity might be affected as the wavefront is affected (generally.)


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Re: First experiments with CO mask new [Re: Asbytec]
      #6105309 - 09/28/13 04:25 AM

Norme, I came across this idea - that CO interacts with seeing to reduce image quality - long ago, in the 1st edition of Jean Texereau's How to Make a Telescope, a copy of which my local library had on the shelves.

More recently, as in 1980s I bought a copy of the 2nd edition, still available I think from Willmann-Bell. From that some quotes:

p.310: ...in reflectors, the central obstruction makes the instrument somewhat more sensitive to turbulence..."
Taking an extreme example, on pp 140-141, where various CO levels are discussed, the case of 0.5 CO: "the first ring is so reinforced that with the slightest added residual zonal aberration, or the least turbulence, it is no longer the radius of the first dark ring, but that of the second which fixes the resolving power. Thus, a 16-inch telescope with an obstruction of 0.5 will hardly be better than an unobstructed 6-inch, and may even be much worse if turbulence increases appreciably...."

Texereau treats turbulence as being similar to defects in the optics, producing reduced wave-front quality in the instrument. Early in the book (p8) he notes that "...A. Danjon has pointed out that under actual conditions [of using the telescope] it is the total wavefront imperfection we must consider: that we must take account of atmospheric disturbances superimposed on the defects of the objective..." Where an objective has defects approaching a quarter-wave [Rayleigh tolerance] "...such an objective is therefore much more sensitive to atmospheric disturbance than an otherwise similar perfect objective...".
A CO is in effect a form of optical imperfection, reducing the wavefront quality. Suiter (Startest) has similar comments.

A further point of interest in Texereau's comments is that better optical quality, other factors being equal, will reduce sensitivity to seeing.


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Re: First experiments with CO mask new [Re: fred1871]
      #6105381 - 09/28/13 07:16 AM

I am not doubting a CO might disrupt the final image, but think it's not so much due to wavefront deformation as it is diffraction - just like the edge of the aperture. A CO is, in effect, a second aperture changing the pattern of diffraction and adding light to the rings much like an aberration. But, unlike an aberration, it does not alter the wavefront. Rather it changes light distribution and slightly alters the location of minima and maxima. Seeing does, of course, alter the wavefront and the combination can reduce peak intensity well below 0.8 normalized. So, in that way, a CO can affect the final image.

To think I had Texereau in my library at one time. Used it to grind my first mirror over 30 years ago. Its interesting he notes the CO can be sensitive to turbulence and the example he gives makes intuitive sense. It appears he's calculating a given amount of irradiance within the second minimum - combining the Airy disc and the first ring into a single unit affecting resolution.

About 83% of the light will be within the second dark space with a CO at 0.5D, so it looks as if he is bringing peak intensity back to 0.8 when discussing the condition. The radius of the 2nd minimum is 2.29 Lambda/D, or about 260/Dmm. For a 16", that's still only 0.64" arc - a tad smaller than a 6" unobstructed scope FWHM (at about 0.74" arc, 113/Dmm if memory serves). Then, if you add some seeing, enough to generate a speckle pattern in the larger scope, it's conceivable that larger speckle pattern FWHM can grow to a 6" unobstructed FWHM pretty easily.

http://www.telescope-optics.net/obstruction.htm

But, that's only if the 6" is not significantly perturbed by seeing, as well. A 6" in D/R0 ~ 2 has an short exposure FWHM right at 0.83" arc (~85/R0mm) vs 0.74" arc in diffraction limited seeing. It is resolving just below the diffraction limit. In this case, a 16" would be at D/R0 ~ 5 with a short exposure FWHM near 0.67" arc (~68/R0mm) and a long exposure FWHM near 1.3" arc (~136/R0mm) or about that of a 4" aperture. (And, of course, hopelessly below the diffraction limit) So, he must be discussing long exposure rather than the lucky snapshot of a larger aperture.

http://www.telescope-optics.net/seeing_error.htm

Note the above figures are based on R0 ~ 102mm dominating seeing, not aperture. This is /generally/ true for any aperture over about 4" in diameter. The figures are based on empirical data, it accounts for FWHM of the seeing affected image - the disrupted speckle pattern. This speckle pattern 'incorporates' the light from the second ring rather than treating it as a separate, clearly seen entity.

But, none of that study differentiates between obstructed and unobstructed scopes. Presumably, they are calculated for unobstructed apertures. One can make the argument an obstructed FWHM might be a few percentage points smaller and less bright. If we're dealing just with the Airy patterns of similar apertures, one might be forgiven for thinking the obstructed aperture will hold it's slight resolution advantage in the same seeing conditions (albeit with a dimmer central disc which might appear to be less than ideal, more mushy resolution of equal double pairs.) Disclaimer: the above paragraph is thinking out loud and speculation on my part.

But, I think this is what Wilfried is seeing, a reduced peak intensity below 0.8 and the appearance of more mushy resolution given seeing with CO approaching and exceeding 0.3D. It's hard to get good peak intensity at larger CO diameters.

So, while the 0.5 CO might add a significant amount of light to the rings, the combined disc/ring diameter is still a function of larger 16" aperture (back to the example above.) Larger aperture, in turn is affected more so by the same seeing conditions. Seeing between long and short exposure of a 16" aperture will bracket performance of a 6" scope - sometimes worse, sometimes better - even with the large CO (given both are of descent peak intensity - including Strehl - to begin with.) However, it makes sense to me the peak intensity of the central disc in the larger obstructed aperture can fall off dramatically as a result of both obstruction and seeing. But, if you add back the increased light within the second minimum of the larger aperture, then you regain peak intensity of that messy speckle pattern...and it's still (sometimes) smaller than a 6" scope in terms of short exposure FWHM.

Sorry for being long, but it's an interesting topic.

Edit: I'm not well versed, but unless the obstruction changes the position of the entrance pupil, the chief ray and OPL should not change. However, such a condition could exist, as I understand it, in a refractor if there is a stop not at the objective (long narrow draw tube, for example) or in a folded instrument with an undersized secondary or tight baffle. Normally, though, the obstruction should not alter the aberration. This was a fear I had moving the aperture stop from the secondary baffle to the meniscus.

Edited by Asbytec (09/28/13 08:14 AM)


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Re: First experiments with CO mask new [Re: Asbytec]
      #6105800 - 09/28/13 11:58 AM

It turns out, Wilfried's unobstructed refractor is putting probably 82% of the light in the Airy disc (assuming a high Strehl of about 0.98.) 98% of 84% available light = 82% in the Airy disc and about 18% in the rings. Refractor peak intensity equals it's Strehl since it has no CO affects, so his peak intensity is 98% normalized.

When he puts a 0.3D obstruction, that changes to about 80% normalized, or a little better than 65% (of the total 84%) in the disc and 35% in the rings compared to 18% when unobstructed - but assuming a very good Strehl of 0.98.

That difference should be noticeable, in accord with his report above, and maybe deteriorate more rapidly as the light falls off to well below 80% normalized. But, I am not sure it's actually affecting resolution, given the seeing conditions...maybe.

Lower frequency contrast is certainly affected, but high frequency contrast should hold up with larger CO size. Maybe only up to a point, though.


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Re: First experiments with CO mask new [Re: Asbytec]
      #6106282 - 09/28/13 04:40 PM

I have meanwhile made a 0.4 CO inset for extended experiments but clear skies with good seeing are currently not available at my location so we have to wait for results to compare good and not so good seeing effects combined with CO.
I will also have special attention on the effect reported from Bill Boublitz in the thread "Separation of Gam Equ" that he could observe a companion sitting in the first ring with the ring brighter than the star due to a visual interruption of the ring - I have assumed so far that such a constellation (ring brighter than secondary sitting in the ring) would make resolution impossible.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6106729 - 09/28/13 11:12 PM

Why not go the full Monty and make a 72% CO as mentioned in Argyles book. Apparently delivers superior results on equal brightness components.

Pete


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Re: First experiments with CO mask new [Re: azure1961p]
      #6106755 - 09/28/13 11:25 PM Attachment (6 downloads)

Quote:

I have assumed so far that such a constellation (ring brighter than secondary sitting in the ring) would make resolution impossible.



I would think so, too, Wilfried. It's an interesting problem to determine the total magnitude of the ring or just the portion of that ring where the companion sits. It does seem if that portion is brighter, then the companion would not peak above it in terms of intensity. The ring would be more intense blotting out the companion.

Even if the companion did 'add' to the intensity, it boils down to the eye's ability to detect irradiance differences. That's exceedingly difficult as Sidgwick mentions. Even the first ring appears brighter than it really is and at some CO levels it appears nearly as bright as the central disc. That's a trick of the eye, as I understand it. Personally, my first ring appears very bright, too, even though it's only about 14% as intense as the central disc.

I would think the companion should be significantly brighter than the ring (either portion or in whole) for it to overcome that brightness and to stimulate the eye's response. It may be true that in modest seeing conditions, where the ring breaks into arcs, the companion might sneak out between the arcs in the rings. Ironically, in good seeing with a more steady first ring, such an observation borders on impossible at some large delta_m.

Quote:

...make a 72% CO as mentioned in Argyles book. Apparently delivers superior results on equal brightness components.



Pete, that's an interesting statement. I would have guessed something beyond 0.5D would still be beneficial while, as Wilfried mentioned above, approaching 0.9D would be closer to the actual limit - nothing but rings. Maybe 72% is some sort of turning point from beneficial to degradation.

Edit: When one plots peak intensity (vertical axis - LHS) as a function of CO (blue curve) vs change in Airy disc diameter (red curve - RHS) also as a function of CO, the graphs intersect at about 70% CO diameter (horizontal axis.) Its interesting, but not sure what to make of it just yet. Strehl ratios are included as shown.

Edited by Asbytec (09/29/13 02:55 AM)


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Re: First experiments with CO mask new [Re: azure1961p]
      #6106885 - 09/29/13 02:54 AM

Quote:

Why not go the full Monty and make a 72% CO as mentioned in Argyles book. Apparently delivers superior results on equal brightness components.

Pete




Pete, with the iris for my C925 I have the full range down to 0.99 CO. While the diffraction pattern with CO larger 0.4 is nice to look at (bulls eye) I found it counter productive for resolving binaries - at least unequal bright ones (my main interest). But even for equal doubles I can not remember any positive effect for resolution - I had even the impression that elongated disks were visually forced perfectly round by the mighty surrounding rings.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6107443 - 09/29/13 12:57 PM

Here's an excerpt from the book just to be clear...
“this last effect is almost negligible for central obstruction much below 50% but it may surprise some readers to learn that for the highest resolution on equal pairs this author deliberately stops out the central 72% of the telescope’s aperture2—a 9-in. central obstruction on a 12.5-in. reflector! Of course, such doubles are extreme high-contrast targets and therefore react quite differently to such treatment, compared with planets or even unequal double stars, whose resolution would be seriously impaired by this tactic.
To bring this discussion to”

Excerpt From: R. W. Argyle. “Observing and Measuring Visual Double Stars.” Springer New York, New York, NY. iBooks.
This material may be protected by copyright.



Pete

Edited by azure1961p (09/29/13 01:03 PM)


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Re: First experiments with CO mask new [Re: azure1961p]
      #6107625 - 09/29/13 02:24 PM

Must be the part of Taylor - interesting but somewhat controversial. Even if true (I don't believe it) it is insignificant as it may be then of use only for a very limited number of equal doubles. And the topic of equal doubles is already covered to full extent in my opinion.
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6107643 - 09/29/13 02:34 PM

Lol I love your honesty Wil - never lose that.

Pete


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Re: First experiments with CO mask new [Re: Asbytec]
      #6107794 - 09/29/13 04:10 PM

Quote:

Quote:

I have assumed so far that such a constellation (ring brighter than secondary sitting in the ring) would make resolution impossible.



I would think so, too, Wilfried. It's an interesting problem to determine the total magnitude of the ring or just the portion of that ring where the companion sits. It does seem if that portion is brighter, then the companion would not peak above it in terms of intensity. The ring would be more intense blotting out the companion.

Even if the companion did 'add' to the intensity, it boils down to the eye's ability to detect irradiance differences. That's exceedingly difficult as Sidgwick mentions. Even the first ring appears brighter than it really is and at some CO levels it appears nearly as bright as the central disc. That's a trick of the eye, as I understand it. Personally, my first ring appears very bright, too, even though it's only about 14% as intense as the central disc.

I would think the companion should be significantly brighter than the ring (either portion or in whole) for it to overcome that brightness and to stimulate the eye's response. It may be true that in modest seeing conditions, where the ring breaks into arcs, the companion might sneak out between the arcs in the rings. Ironically, in good seeing with a more steady first ring, such an observation borders on impossible at some large delta_m.

Quote:

...make a 72% CO as mentioned in Argyles book. Apparently delivers superior results on equal brightness components.



Pete, that's an interesting statement. I would have guessed something beyond 0.5D would still be beneficial while, as Wilfried mentioned above, approaching 0.9D would be closer to the actual limit - nothing but rings. Maybe 72% is some sort of turning point from beneficial to degradation.

Edit: When one plots peak intensity (vertical axis - LHS) as a function of CO (blue curve) vs change in Airy disc diameter (red curve - RHS) also as a function of CO, the graphs intersect at about 70% CO diameter (horizontal axis.) Its interesting, but not sure what to make of it just yet. Strehl ratios are included as shown.




So that they cross at 70% is indicative of max high frequency resolution for an obstructed aperture? Its interesting Im trying to work out what Im seeing.

Pete


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Re: First experiments with CO mask new [Re: azure1961p]
      #6108388 - 09/29/13 10:55 PM

Quote:

So that they cross at 70% is indicative of max high frequency resolution for an obstructed aperture? Its interesting Im trying to work out what Im seeing.



I am not sure, Pete. But, normally if you want to find an optimum between two variables, you find their intersection of a graph. That they intersect at 70% CO and only 25% peak intensity is interesting. It seems to confirm Argyle's statement. There may be something to that, but it isn't matching up with Wilfried's observation. Yet it's fairly consistent with a 40% CO being optimized for the entire MTF.

At 70% CO, it may be optimal for the right hand side of the MTF, I dunno. I optimized only one variable trying to maximize peak intensity toward the Raleigh criterion - not diminish it for the sake of resolution. Though, maybe the figures suggest it can be done closer to 70% CO.

It's not clear how "pleasing" such a dim central disc would be. But, I disagree with Wilfried that a 25% reduction in Airy disc is insignificant, its the difference between 0.92" and 0.69" for a 6" aperture - if seeing permits one to actually achieve that level of resolution and despite the "ugly" low peak intensity required.

Edited by Asbytec (09/29/13 11:29 PM)


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Re: First experiments with CO mask new [Re: Asbytec]
      #6108502 - 09/30/13 12:28 AM

Nice thing about the 72% CO is its so cheap and easy to test in our own scopes. If nothing else it'll look wild Ill say that but I have faith in Lords experiences and your interpretations here. It could prove to be an invaluable tool for doubles fitting a specific brightness, equality in brightness and sub second seperations where anything at all to increase the odds of seeing a narrowing or notch between disks is welcomed. Perhaps it could turn an oblate appearing star into a more elongated shape? Interesting tools for exceptional tasks!


Pete


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Re: First experiments with CO mask new [Re: azure1961p]
      #6108634 - 09/30/13 03:15 AM Attachment (10 downloads)

I propose that you (Pete and Norme) simply try it - in my opinion there is nothing such convincing than own first hand experience. Before doing my own experiments with huge CO values I was like you according to diffraction theory of the opinion that the reduction of the size of the Airy disk is even with large CO values of increasing benefit for resolving close doubles - now I am sure it is not.
See the graph - which image would you prefer?
Wilfried


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Re: First experiments with CO mask new [Re: WRAK]
      #6108731 - 09/30/13 06:47 AM

I think we'd all prefer the image on the left, it's the reason I went with a smaller CO than a larger one. I may throw a 70% CO later in the observing season to have a look see.

No doubt the highly obstructed images are not preferred. I don't understand why they would not perform in a specialized way on tight doubles, unless peak intensity falls off so dramatically the central disc becomes "useless" in some way.


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Re: First experiments with CO mask new [Re: Asbytec]
      #6108864 - 09/30/13 09:02 AM

Well the overall winner here is the one in he left but the simple fact is the central disc is smaller on the right even if the stars would look ugly.

Pete


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Re: First experiments with CO mask new [Re: WRAK]
      #6110016 - 09/30/13 08:55 PM

Wilfried, I'll change the subject back to small - rather than huge - CO effects, to ask a question.

You've mentioned you have a 200mm Dall-Kirkham with 0.25CO on order; how much improvement do you expect on unequal pairs with that, compared to the unobstructed 140mm and 0.38CO 235mm? What are their relative rankings on your current formula?

I'm now looking at getting a larger reflector with small CO, and obviously a 254mm (10-inch) with 0.25 CO would do better than 235mm with 0.38 CO - again, by how much?

Are you able to provide some numbers for ranking these various optical configurations for observing unequal doubles?
Thanks in advance for any light you can shed on this ....


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Re: First experiments with CO mask new [Re: fred1871]
      #6110584 - 10/01/13 04:58 AM Attachment (8 downloads)

Fred, CO influence is currently the weakest part of the current algorithm I fear - I have implemented the positive effects of reduced Airy disk but without threshold (now I think there is a threshold at 0.35CO) and the negative effects for unequal binaries based on statistical analysis but with a less than sufficient data base. Result are therefore overly optimistic results for larger CO values I think. So I hesitate do give precise numbers, as they have to be wrong:
- 235mm with 0.38CO should resolve 1" +4.7/8.9mag - this is too optimistic I think
- 254mm with 0.25CO should resolve 1" +4.7/9.2mag - this may be a tad more realistic
- 254mm with zero CO should resolve 1" +4.7/9.4mag - this should be doable rather easily I think.
For a visual comparison see the graph - again: too optimistic for 0.38CO I think now, never better than zero CO and much worse with increasing delta_m.

What I expect from my hopefully within a reasonable time frame coming 0.25 CO 200mm DK is 200mm refractor like performance for equal binaries (with less CO about ~0.2 I would even hope for a slight advantage compared to a refractor) and decreasing performance down to an equivalent of ~180mm for unequal binaries.
For 254mm 0.25CO I would expect equivalent relative performance with some degradation depending on seeing.

Wilfried


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fred1871
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Re: First experiments with CO mask new [Re: WRAK]
      #6112076 - 10/01/13 10:02 PM

Wilfried, thanks for the information - helpful, even though you feel there's much more to be done on this topic. Looking at your list of delta-m at 1" separation, I feel also that the effect of a larger CO, such as 0.38, is underestimated.

My observing experience has been that for typical close pairs with delta-m around 2 magnitudes or more, my 140mm zero-CO refractor does just about as well as my 235mm 0.38-CO SCT. That could indicate I've not yet pushed the SCT hard enough; entirely possible. I have observed various unequal close pairs with the SCT, thought these doubles difficult, then later looked at them with the refractor and could see them with that - and usually at similar magnification. Perhaps the refractor has a magic spell on it already ; it has enabled me to exceed what I thought possible with that aperture.

Looking at your graph, it appears that with your current limited data, there's more loss in going from zero-CO to 0.25-CO, than in going from 0.25-CO to 0.38-CO. That surprises me; intuitively, and from long-time observing, I'd expected the deterioration from 0.25 to 0.38 to be larger. So I ran the numbers on the amount of light in the first diffraction ring - the critical element here, I'd expect. Rounded, the figures for the 1st diffraction ring are:
zero-CO 1st ring 7% disc 84%
0.17-CO 1st ring 13% disc 78%
0.25-CO 1st ring 18% disc 72%
0.38-CO 1st ring 28% disc 60%
So that would support the graph as being in the right direction, though it doesn't fit well with my sense of how it works in practice, based on observing with various obstructed scopes including Newtonians, in the range of 0.24-CO to 0.38-CO. (I've not had the pleasure of observing with a 0.17-CO telescope.)

Or perhaps the issue is relative brightness (and spread) of the diffraction ring compared to the disc - at 0.17-CO the ring has 1/6 as much light as the disc; at 0.25-CO the ring has 1/4 as much light as the disc; at 0.38-CO it has nearly 1/2 as much light as the disc.

Your suggestion that the 200mm DK (0.25 CO)might be similar to ~180mm unobstructed, for unequal binaries, is of interest; that's something that could be checked, as a fractional loss of aperture, by using a 0.25 CO on your refractor. Does it, with 0.25 CO in place, behave like an aperture of 126mm? Of course, that's not a huge difference, and seeing conditions can have a bigger effect than that.

Your list of doubles observed with various CO levels, that you put on this thread a week ago, had only one example with significant delta-m - STF 2616, delta-m 2.8, at 3.3". In that example, the secondary star, apparently easy at zero-CO and not difficult at 0.15-CO, became very difficult at 0.25-CO, and nearly invisible at 0.35-CO. Because the seeing that night was not good it might not give results indicating what's possible with good seeing. But it does suggest that 0.25-CO does not compare well with 0.15-CO or zero-CO; and that 0.35, while worse than 0.25, shows less deterioration than might be expected.

So, it's back to the telescope, and discovering what 0.38-CO will allow to be possible. I've been making a list of the more difficult unequal pairs I've already successfully observed with the SCT; and I'll now go hunting for more of them that might be testing for it.

And, at the moment, I'm feeling less certain that moving from a scope with 0.38-CO to one with 0.25-CO will make a large enough difference to be worth the cost and effort. Factoring in better optics, plus more aperture, in addition to reduced CO, might make it worthwhile. Hmmm... time for further studies before committing to a replacement for the 235mm (which is a good example of the type).

Edited by fred1871 (10/03/13 03:36 AM)


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Next experiments with CO mask new [Re: fred1871]
      #6114604 - 10/03/13 06:23 AM

From my last session in Equ with seeing ~6 Pickering and altitude less than 40° - not perfect conditions but better than last time:
01.02 STF2765 (HIP 104570) – 2.8"DS +8.47/8.50mag:
0.1: no significant change compared to zero CO
0.15: same
0.2: Image tad crisper
0.25: no significant change
0.3: no significant change
0.35: bit blurred, slightly worse than zero CO
0.4: no longer clear split, only with moving the target through the field of view
With CO 0.5 I would expect no longer resolution. Will make an additional inset

01.08 STF2786 (HIP 105295) – 2.8"DS +7.49/8.2mag:
0.1: no significant change
0.15: image tad crisper
0.2: no significant change
0.25: no significant change
0.3: tad fuzzy, similar to zero CO
0.35: more fuzziness, worse than zero CO
0.4: no longer stable split, but still resolution

01.11 BU163 (HIP 105200) – 0.9"DS +7.31/8.88mag:
0.1: no significant change
0.15: no significant change
0.2: first hints for split/notched Rod
0.25: for short moments clear hint of split
0.3: rod
0.35: tad fuzzy rod similar to zero CO
0.4: image worse than with zero CO.
Very interesting result - first time real experience of better resolution due to small CO compared with zero CO.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6114649 - 10/03/13 07:39 AM

Wilfried

Very interesting indeed on BU163.

Roberto


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azure1961p
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Re: Next experiments with CO mask new [Re: R Botero]
      #6115063 - 10/03/13 12:28 PM

Wilfried your finds are a little puzzling. You and Fred hold the Argyle book in high regard and even slanted doubt on my finds that broke form with the book . Then here we have you doing a full about face stating that Lords is wrong and a 72% CO can only be detrimental much to the esteemed author claiming otherwise. I mean which is it Wil? Lord is right so long as Im wrong or Lord is wrong because his finds don't jive with your results?
Heres what I am suspecting your true finds are revealing. These results keep Lord in the good stead you and Fred hold him in and at the same time you are seen as an exquisite observer. Here it is:


It sounds like your application of central obstructions is covering out the better part of your objective. You probably have zones of inaccuracies that the CO brings to clear definition by way of the degrading effect it has on the diffraction pattern. You may have a clunker of a refractor. At any rate I did a 50% CO obstruction on my reflector and aside from the ghastly bright rings the spurious discs of Epsilon Lyrae were superb.

You may find your tests are more fruitful by masking off the outter inch or so of your refractor.

Lord would do no less.


Pete
Ps: keep your chin up your getting the 200mm.

Edited by azure1961p (10/03/13 10:42 PM)


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6115772 - 10/03/13 05:53 PM

Pete, I am not sure what your point is - the higher CO the better resolution?
I also do not understand your reference to Lord - I don't know of any experiments of Lord with size of CO. But in his Rule of Thumb algorithm for resolving unequal double stars he uses CO as parameter besides aperture, seeing, separation and delta_m. For example his result for BU163 0.9" +7.31/8.88mag for a 140mm refractor with fair seeing (Pickering 5) is "eXeedingly Difficult" and with a 140mm reflector with 0.4 CO it is "UNresolvable".
I also do not understand your argument with 0.5 CO on your 200mm reflector - aperture rules and you have still an photon sampling surface equivalent to an 173mm refractor so why should the spurious disk not be still rather crisp. The relevant question here is only if 0.5 CO enhances your chance for resolving doubles compared with your standard CO. If you feel this is the case then you should stay with 0.5 CO and never look back.
Finally - I am not in possession of truth and look only for obvious evidence and I am so far rather satisfied with my findings: small CO enhances somewhat the resolution (especially for equal doubles) and large CO leads somewhat to degradation (especially for unequal doubles). That above of this CO effect aperture still rules at least to some degree is anyway not in discussion - a 200mm scope with 0.5 CO will certainly resolve more doubles than a 60mm scope with zero CO.
Wilfried


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6115783 - 10/03/13 06:01 PM

It's curious he is generally seeing degraded images at 30% and up. I still think this is due to reduced peak intensity where above 0.3D it falls off below 0.8 - the peak intensity related to the minimal Raleigh criterion for 1/4 P-V performance. There is probably no coincidence between what he is seeing and the optimum CO for an instrument with a very good Strehl.

Of course the unobstructed peak intensity is equal to the Strehl, so the Airy disc is also 10% larger as well as closer to 95%, or better, normalized peak intensity. In other words, he is seeing 95% of the 84% light available to the central disc whereas at 30% obstructed he is only seeing 80% of the 84% available.

For BU163 (0.9" mag7.31/8.88) he note's hints of a split at 0.25D back to a rod at 0.3D, then worse at larger COs. This seems to suggest the central disc is getting larger above 0.25D and that should not be correct. FWHM in coherent light should continue to fall off with increasing CO. As I understand it, light is pretty much coherent in modest amateur scopes. Even if not, FWHM still falls of quite a bit at COs larger than 0.4D which should allow better resolution.

So, there must be some interplay between the modest seeing and peak intensity in interpreting his results.


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6115829 - 10/03/13 06:30 PM

Nice analysis Norme. I'd be interested to see the paper on the optics first though. Its not following form as Id understand CO effects so its a curious thing anyway. I'd be interested to see his instrument on order perform this test.

A little sobering that the effective CO then on the iOptron 150 Mak i was intendibg to buy is a whopping 41%. Wether or not my C6 37% CO is noticeably better I may never know - at least it doesn't start with a four!

Anyway Im drafting up a list of doubles for the Celestron. It'll be interesting to see what happens at the Dawes and so on. Last double I looked at through it was Porrima in Virgo. All CO contentions aside it was a beautiful view through that scope. I can't say Ive compared it to a 6" unobstructed ever but I can't fault the clarity of the pattern - bright rings and all.

Pete

Edited by azure1961p (10/03/13 06:31 PM)


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6115850 - 10/03/13 06:45 PM

Pete, zero CO seems optimum for the left hand side of the MTF where larger contrast detail is found but at the expense of higher frequency resolution. This is why we see the obstructed MTF curve drop below that of a perfect same aperture and why it takes a larger obstructed aperture to match the contrast transfer of a refractor. This is the realm of unequal pairs with a separation wider than the first ring or so.

With increasing CO up to about 30%, it tends to be optimum for higher frequency details such as close doubles while maintaining peak intensity above 80% depending on Strehl. Larger than that, and you get better optimization for high frequency at the expense of peak intensity and lower frequency contrast. This is why, at about the Airy disc dimensions at the very high frequency end, the MTF for an obstructed scope is higher than that of an unobstructed scope. This is the realm of close equal pairs.

Right around 40%, you get an optimum performance along the entire MTF with better performance at high frequencies at the expense of 'refractor-like' performance at lower frequencies due to increasing ring brightness. There is some sort of balanced performance across the entire MTF as well as versatility in the design. Low frequency performance falls off at even larger CO diameters, but it is supposed to be somewhat better at the high frequency end.

http://www.telescope-optics.net/obstruction.htm


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6116061 - 10/03/13 09:04 PM

Quote:

It sounds like your application of central obstructions is covering out the better part of your objective. You probably have zones of inaccuracies that the CO brings to clear definition by way of the degrading effect it has on the diffraction pattern. You may have a clunker of a refractor. At any rate I did a 50% CO obstruction on my reflector and aside from the ghastly bright rings the spurious discs of Epsilon Lyrae were superb.

You may find your tests are more fruitful by masking off the outter inch or so of your refractor.

Lords would do no less.

Pete





Pete, you're making too many assumptions about optics - in particular, assuming Wilfried's refractor has the equivalent of TDE. And there can be zones in the central part of an objective, or mirror; not to say that it's the case for Wilfried's telescope. The results he's had using the scope unobstructed show that it's good, when seeing allows.

TDE is far more likely a problem in final image quality with reflectors than refractors.

Regarding Chris Lord, I don't believe that he's the last word on things - but he has done a useful study, and some of the data and other information is helpful in doing further assessments in this area.

There are various issues in the recent discussion (from several people) that I want to talk about, but it'll have to wait until later today as I have a meeting to go to. However I thought your note needed a first reply from me as well as from Wilfried (and I'm glad to see he has commented here).


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Re: Next experiments with CO mask new [Re: fred1871]
      #6116152 - 10/03/13 09:44 PM

True enough Fred however more than one refractor has improved its image due to stopping down the aperture and as a result of course increasing FL.
I am aware of TDEs being often a reflector issue though. Still the falling apart of the image at a 40% CO still seems peculiar. I'm getting Normes take here - optimized for high end frequency not low contrast and a trade off of peak intensity but how does that explain the pattern falling apart like that on doubles? I can see the large CO modifying the pattern but not destroying high frequency contrast like the classic Dawes notch and such.
Its as though there's this smoothing taking place .

As an aside, I'm glad you are getting a ten inch F/7. Its actually got a nice edge over the 8" I've seen in side by sides. More so than Id have guessed
even.

Pete


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Re: Next experiments with CO mask new [Re: WRAK]
      #6116229 - 10/03/13 10:30 PM

Quote:

Pete, I am not sure what your point is - the higher CO the better resolution?
I also do not understand your reference to Lord - I don't know of any experiments of Lord with size of CO. But in his Rule of Thumb algorithm for resolving unequal double stars he uses CO as parameter besides aperture, seeing, separation and delta_m. For example his result for BU163 0.9" +7.31/8.88mag for a 140mm refractor with fair seeing (Pickering 5) is "eXeedingly Difficult" and with a 140mm reflector with 0.4 CO it is "UNresolvable".
I also do not understand your argument with 0.5 CO on your 200mm reflector - aperture rules and you have still an photon sampling surface equivalent to an 173mm refractor so why should the spurious disk not be still rather crisp. The relevant question here is only if 0.5 CO enhances your chance for resolving doubles compared with your standard CO. If you feel this is the case then you should stay with 0.5 CO and never look back.
Finally - I am not in possession of truth and look only for obvious evidence and I am so far rather satisfied with my findings: small CO enhances somewhat the resolution (especially for equal doubles) and large CO leads somewhat to degradation (especially for unequal doubles). That above of this CO effect aperture still rules at least to some degree is anyway not in discussion - a 200mm scope with 0.5 CO will certainly resolve more doubles than a 60mm scope with zero CO.
Wilfried




Wil I missed your post in the thread. Ill hit on a couple things...

I posted a qoute from Lord some time back stating his advantageous use of a 72% CO which was something like 9" covering a center on his 12" reflector. He had infact experimented one would assume in arriving at this satisfying percentage. I would imagine somewhere along the line 75% had diminishing returns or no improvement to say nothing of too much dimming.
But the man has used a large CO to improve his views and in the face if that you essentially scoffed at it. I can cut and paste from his book again if need be.

Lastly, 8" doesn't thrive on a 50% CO . Its works through an exception to the rule on a narrow list of applicable doubles . The 16% CO is the winner here across the board but the effects of a large CO are at least curious. Again, on doubles of a specific seperation bracket with not too much magnitude difference. Alas Lord doesn't give an algorithm formula that worked with his huge CO - it could be that your components are too far apart in brightness for the 72%? In that case it would not indicate defects in your objective but merely the need for a more suitable subject.

My point Wil was simply that you were giving me two different takes on Lord. I have difficulty with him making this wonderful algorithm then derailing himself into the unbelievable with the 72%. It more than likely has a very lean diet of acceptable pairs. Your reversible on his credibility in this claim was a little surprising and it raised questions on your finds.

Its be a fine thing to see what ROT supports his CO claim.

Pete

Edited by azure1961p (10/03/13 11:04 PM)


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Re: Next experiments with CO mask new [Re: fred1871]
      #6116411 - 10/04/13 12:14 AM

Quote:

...refractor has the equivalent of TDE. And there can be zones in the central part of an objective...




In my understanding, there may be the equivalent of a TDE if a very good refractor balances HSA by adding a touch of LSA of opposite sign. Balancing 6th/4th order SA requires a bit of turning back of the edge, and I'm assuming good refractors take the time to achieve that. There is nothing wrong with Wilfried's optics, surely, to explain his results.

However, rather than hamper, this balancing low/high order SA improves performance from the edge leaving the central portion of the lens with a zone. In a Mak, also with highly curved surfaces and residual HSA - truely 'refractor-like' in that sense, the offending central zone is masked improving RMS. This is one reason Roland got on the soapbox over the star test.

http://geogdata.csun.edu/~voltaire/roland/startest2.html

Wilfried is doing the same by masking off the longer focal length of the center. He's probably improving RMS somewhat while loosing peak intensity using an obstruction. This is what is so curious about his findings above 0.25 to 0.3D leaving me to believe it's peak intensity and seeing causing the image to degrade at those levels.

He /should/ be getting slightly better resolution on tight pairs. If he's not, then moderate seeing may be influencing his RoT which might actually be possible in diffraction limited seeing at and better than 8/10 Pickering.


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Re: Next experiments with CO mask new [Re: WRAK]
      #6116498 - 10/04/13 02:10 AM

Wilfried, even with the small number of pairs you've tried with various levels of CO, I'm seeing some possible indications of the effect of CO, according to size, interacting with delta-m as a second variable, and not in the straightforward way that might be expected.

First, the data. Your earlier list suggested a slightly crisper image with CO-0.15, on the near-equal pairs (STF 2166, Pi Aql, A 1663).

With STF 2616, delta-m 2.8, CO-0.15 made the companion dimmer, and larger CO sizes made it harder and harder to see.

From the new list, again there's a slightly crisper image (STF 2765), this time with CO-0.20; and with STF 2786 at CO-0.15. Both are near equal, though STF 2786 is a little off with Dm of 0.7.

But with BU 163, delta-m ~1.6, there's a surprising difference - CO in the 0.20 to 0.25 range appears to have given the best image, better than CO from 0.0 to 0.15.

In all cases, it appears that large CO of 0.35-0.40 is less good than moderate CO in the 0.15 to 0.25 range.

One possibility to consider is that small-ish CO may be helpful for moderate Dm differences, such as the Dm 1.6 of BU 163; give slightly crisper images with minimal Dm; and be unhelpful with larger Dm.

Obviously a lot more data points are required before reaching any definite conclusions. But as a preliminary idea, to be investigated further, there's a possible usefulness of smaller CO with pairs that are not too unequal - previous ideas suggested there's a linear relationship of CO to Dm factors - increased CO helps equal pairs by making the disc smaller; then CO is unhelpful as Dm increases from about 1-magnitude. Perhaps, instead, moderate CO (0.15-0.25) can be helpful in the Dm 1.0-2.0 range (or thereabouts), though becoming unhelpful again somewhere around or beyond Dm 2.

We'll see what shows up in the data as you continue the CO experiments.


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6116529 - 10/04/13 03:22 AM

Quote:

True enough Fred however more than one refractor has improved its image due to stopping down the aperture and as a result of course increasing FL.
I am aware of TDEs being often a reflector issue though. Still the falling apart of the image at a 40% CO still seems peculiar. I'm getting Normes take here - optimized for high end frequency not low contrast and a trade off of peak intensity but how does that explain the pattern falling apart like that on doubles? I can see the large CO modifying the pattern but not destroying high frequency contrast like the classic Dawes notch and such.
Its as though there's this smoothing taking place .

As an aside, I'm glad you are getting a ten inch F/7. Its actually got a nice edge over the 8" I've seen in side by sides. More so than Id have guessed
even.

Pete




Well, increasing the focal ratio has the benefit of improving how eyepieces work. And, agreed, if a refractor has a faulty objective, masking off areas can help. Ditto reflectors.

"the falling apart of the image at a 40% CO" - I'd put it down to a mix of diffraction effects with too much light in the rings, interacting with the seeing (as in Texereau) - so there's smearing of light from the rings. The result is a messy image, and if the optics are only middling quality, then more light in rings plus seeing effects will make optical imperfections more obvious by addition. This will be especially obvious with unequal pairs. But my impression is that Wilfried's scope has high grade optics, and I'd think diffraction from a large CO plus not-good seeing would be enough to mess things up.

I'm now looking again at Wilfried's way of putting a CO in front of his refractor - those two bars, and not overly thin at that - they'll have a further diffraction effect, and a non-symmetrical one. That will change the diffraction pattern in a different way from the central disc, and the effect will be superimposed on the effect of the CO disc.

I'm now wondering if the effects noted by Wilfried are partly a result of the support arms for the CO, which are different from the usual Newtonian systems of 3 or 4 vane spiders.

Regarding my getting a ten-inch - yes, I'm planning to decide on a version of that aperture soon. But I might get a Dall-Kirkham instead of a Newt, so an even longer f-ratio, as I prefer Cassegrain-style scopes for long f-ratio (no ladders, less bulky/unweildy, etc). But small CO (0.25 at largest) it will be.


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Re: Next experiments with CO mask new [Re: fred1871]
      #6116533 - 10/04/13 03:32 AM

Quote:

...previous ideas suggested there's a linear relationship of CO to Dm factors - increased CO helps equal pairs by making the disc smaller; then CO is unhelpful as Dm increases from about 1-magnitude.

...made the companion dimmer, and larger CO sizes made it harder and harder to see.

In all cases, it appears that large CO of 0.35-0.40 is less good than moderate CO in the 0.15 to 0.25 range.



That pretty much sums up those "previous ideas" where peak intensity falls off with CO diameter but can remain "crisp" (presumably that means brighter) with a moderate CO. It speaks to the idea resolution may be occurring in the pure sense, but perception is failing empirically because of the fall off in peak intensity. It may be hampered further by modest seeing, where the visible disc is small compared to seeing error.

This dimming would seem to affect dimmer stars at larger delta_m, "making them harder to see." A dim star may compete with some brightness gradient from the primary, especially those on the first ring where eye response (irradiance) plays a role, as well. In a previous thread you gave the nominal brightness for the disc and the first ring, but not their apparent brightness due to our eye response.

All of this seems to be offering empirical results that are consistent with the observer's eye response, seeing, and co affects but does not disprove resolution is occurring with larger CO. It may just require diffraction limited seeing to show it.

When you have a very close pair, the ring structure is well outside the discs. That he goes from a possible split back to a rod shape is telling. It could be the rings wash over the entire image in some random way, but to loose the split means something else is at play. I doubt it's optics, and it may not be the dancing ring structure found outside that rod formation. It has to be perception as his observations are empirical in nature - again - probably affected by seeing when the disrupted visible disc (not so much the brighter ring) is small (and dimmer) compared to seeing error. The same thing happens with larger aperture, the visible disc is small compared to seeing error disrupting the visible disc itself.


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6116540 - 10/04/13 03:41 AM

Quote:

... This seems to suggest the central disc is getting larger above 0.25D and that should not be correct...




Norme, this is certainly not what I suggest - but as I do not change magnification when changing CO size the diffraction pattern begins to fill up the gap with higher CO. Probably with higher magnification (if scope and seeing permits it) the split would be obvious again. But this does not change the described process of degradation but only shifts it to a next level of higher aperture - looking at the same double with a CO of for example 0.35 may mean for the 140mm scope "not resolvable" and for the 200mm "easy". So aperture wins but the game is repeating when stepping up to a more demanding double - then the 200mm gets "non resolvable" and the 300mm gets "easy" (if seeing permits).
So effects of CO cannot be separated from other resolution influencing factors.
Wilfried


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6116545 - 10/04/13 03:46 AM

Norme, balancing aberrations in a refractor is very different from doing it in a maksutov. That's why the star test works differently for those designs, and why Roland wrote the piece he did - but I think it was quite a long time ago, and he's referring to the 1st edition of Suiter's book. Suiter added new material, including discussion specific to maks for evaluating optical quality via the star test, for the 2nd edition.

Re Wilfried's experiments - yes, he will lose peak intensity, inevitably given the presence of CO, and the contrast of disc and rings will change; and seeing will have its effect on top of that; but Suiter also comments on CO having far more serious effects once you get to 0.3 and beyond. And that's on the image as it were in a vacuum, separately from seeing defect additions.


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6116555 - 10/04/13 04:07 AM

Quote:

... I posted a qoute from Lord some time back stating his advantageous use of a 72% CO which was something like 9" covering a center on his 12" reflector...

Its be a fine thing to see what ROT supports his CO claim.
Pete




Pete, then this is a misunderstanding. I have Argyle not at hand now but I think you refer to the part of Taylor - Lord wrote nothing for Argyle as far as I can remember.
Regarding Taylor - I found his article very interesting but at the same time somewhat overeager in arguing pro reflector for resolving doubles. But now I see that he has made good points especially for resolving equal doubles with Newtons with small CO.
I remember the claim for enhanced resolution with increased CO starting from a rather small starting point - this seems correct to a certain degree but I still consider the extreme claim for 0.7 CO as helpful for resolving anything dubious. Maybe there is a special case for a very limited number of doubles but then this is not of use for any RoT as the rule has to be covered and not the exception.
And Taylor concentrates on rather equal doubles and resolving unequal doubles is quite a different challenge with CO getting obviously less and less helpful but more and more destructive.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6116558 - 10/04/13 04:24 AM

Wilfried, it's an interesting aspect you bring to light. No doubt increasing aperture would be beneficial to resolution on a given separation and provided the aperture (plus CO affect) does not drag the first ring directly over the companion.

I am having a hard time understanding what it means when you say the diffraction pattern "begins to fill up the gap" turning a possible split back into a rod formation. If this is happening on a tight pair, that means the first bright ring is some how interfering across the very close split between two Airy discs.

What I am saying, and trying to be helpful based on my understanding of CO affects and seeing, is it may be the dimmer visible disc itself that is disturbed in modest seeing and closing the gap it forms with the primary. It should be compounded by the fact it is dimmed by the CO and made more difficult to discern at whatever constant magnification you are using.

That may explain a reduction in visual resolution from possible split to rod shape with increasing CO. You're correct, the central disc should not be getting larger with increasing CO, but it can with seeing especially when it is small compared to the seeing error induced.

For example, at clear aperture a 140mm refractor has a Raleigh criterion of 0.99" arc and a FWHM of ~0.8" arc (if memory serves.) At a CO of 30%, the Airy disc is ~10% smaller and FWHM falls from 1.028 Lambda/D to 0.97 Lambda/D. That's 0.83" arc and 0.78" arc respectively making the dim companion's visible disc not only dimmer and smaller but also more susceptible to a given seeing error. In fact, both visible discs would be more susceptible seeing with decreasing FWHM.

The same thing happens with smaller visible discs with increased aperture - which is more sustainable to seeing for the same reason: greater error and smaller disc. A smaller aperture can sustain more pleasant images in the same conditions: less error relative to the larger visible disc equals a "crisp" - possibly diffraction limited - image.

Anyway, you re getting empirical results that differ from theory and I am sure it's not your optics. And at such tight Dawes or Raleigh separations, it seems the brighter first ring is well enough outside that inner nucleus as not to be a major factor unless you are at relatively low magnification. But, you're apparently high enough to discern a rod formation. It has to be something related to the 6/10 seeing - well beyond diffraction limited - and CO dimming affects.

If your seeing were truly limited by diffraction, then the (more lab like) conditions may show resolution missing above and offer more lab-like results.


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Re: Next experiments with CO mask new [Re: fred1871]
      #6116561 - 10/04/13 04:34 AM

Fred, thanks, I do not have Suiter's second edition but would love to know what he updated concerning Mak star testing and balanced aberrations. I am evaluating my own scope to see if the balanced form was used. It's exceedingly difficult to star test for. In any case, my own star tests are more like those Roland discusses, they are not symmetrical either side of focus but the in focus images remain well defined and in focus diffraction is "clean."

So, without belaboring the point and dragging it off topic, I just wanted to put to rest the idea Wilfried's optics are playing a role in his observations.

I am curious, though, what "serious effects" Suiter addresses at CO above 30%. The results will not be in a vacuum, but include all of those effects you mention. I hope that is what I am driving at despite my ignorance of anything additional Suiter might have to say.


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6116565 - 10/04/13 04:53 AM

Last night again clear sky but seeing somewhat worse than the night before, rather Pickering ~5 and less. To add insult to injury an aircraft crossed my field of view making the seeing even worse for some minutes.
Tried BU838 1.6" +7.92/10.02mag with my 140mm in Equ and did not get a clear split but only a hint at 5 o'clock - current RoT suggests 124mm for BU838 so this indicates really bad seeing.
I then applied the CO mask:
0.1 - no change
0.15 - split for fractions of seconds
0.2 - same with more wobbling in seeing
0.25 - similar to zero CO
0.3 - begin of degradation.

This is quite interesting:
- Advantage of small CO seems independent of seeing
- Advantage of small CO works obviously also for unequal pairs at least up to delta_m of 2 or even more.
Wilfried


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6116584 - 10/04/13 05:33 AM

Quote:

... I am having a hard time understanding what it means when you say the diffraction pattern "begins to fill up the gap" turning a possible split back into a rod formation...




Pete, this is simply a question of magnification - if you have a split with a not this high magnification you see only the spurious disk and not the diffraction rings or may be the first diffraction ring directly attached to the spurious disk. If with increasing CO the energy is transferred from the spurious disk to the rings then the image of the disks gets bloated and blurred with unchanged magnification and a clear split may then change to a rod. At least this is what i have observed.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6116609 - 10/04/13 06:26 AM

Since this is interesting gonna crunch the seeing numbers based on empirical data on seeing. In 6/10 Pickering with 140mm aperture, this implies D/R0 of about 1.75 on average or R0 ~80mm. Short exposure FWHM should be close to 0.75 Lambda/R0 (85/R0) or about 1.1" arc as opposed to diffraction limited (1.028 Lambda/D) 0.83" arc. And worse, of course, for long exposures which we're not using.

An aperture that gives 0.78" arc FWHM (same as 0.3D obstructed 140mm aperture) is 116.6/0.78 ~ 149mm. Its and /effective/ increase in clear aperture producing a smaller FWHM that would be susceptible to seeing at that aperture. It's the FWHM that is more important, I think, than the minor /effective/ increase in aperture. The larger effective objective (smaller FWHM), though, nudges D/R0 a tiny bit closer to 2 and decreases slightly the short exposure FWHM to, say, 0.7 Lambda/R0 or about 79/R0 ~0.99" arc. That's enough for a Raleigh split or at least a dark space with an obstructed 140mm clear aperture in 6/10 short exposure seeing - and a clear 140mm aperture in diffraction limited seeing.

If the logic is correct, then CO induced FWHM should improve given the seeing from just beyond a Raleigh split with FWHM at 1.1" to just right at Raleigh with 0.99" arc - the difference between 141.6/Dmm and 138.4/Dmm. This happens because of a slight decrease in FWHM. At some point, the FWHM is equivalent to a larger "effective" aperture and seeing eventually degrades and short exposure FWHM expands closing the gap and returning to a rod shape. The same effect is probably happening closer to Dawes, as well. It's hard to say at what point this expansion happens (D/R0~3 or more), but I think this is what Wilfried is seeing - pun intended - and reporting.

That small amount of FWHM change might be just enough to open up a tiny gap at some point as the FWHM becomes small. It could be reducing FWHM to 0.78" arc using a CO might open up a small gap between the discs, then close that gap as seeing error relative to disc size increases /effectively/ pushing D/R0 to the point where FWHM no longer decreases. This effect is very real even if minuscule...that's the realm we're operating in.

Coffee is cold...anyway, it was a fun exercise. Might outta do a graph of decreasing FWHM with CO and increasing FWHM with Lambda/R0 and see where they meet. Its pretty complicated and I'm not sure of the logic affecting D/R0. And there is plenty of room for error when working in such tight spaces and small decimal places.

I do not doubt Wilfried's observations, just trying to understand them.

Quote:

...this is simply a question of magnification - if you have a split with a not this high magnification you see only the spurious disk and not the diffraction rings or may be the first diffraction ring directly attached to the spurious disk.



So, you're using just enough magnification to suit your acuity?

Edited by Asbytec (10/04/13 08:45 AM)


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Re: Next experiments with CO mask new [Re: fred1871]
      #6117155 - 10/04/13 12:15 PM

Quote:

... I'm now looking again at Wilfried's way of putting a CO in front of his refractor - those two bars, and not overly thin at that - they'll have a further diffraction effect, and a non-symmetrical one. That will change the diffraction pattern in a different way from the central disc, and the effect will be superimposed on the effect of the CO disc...




Fred, this is a concern I had myself too. Although the used bar is less than 1mm thick I expected at least some spikes with brighter stars from it but so far I did notice none if it - most probably because most of the doubles I observe are not this bright. And it is really amazing that you see virtually no degradation of the image at all when putting the mask on the scope and also with the 0.1 CO inset as this is completely counter intuitive.
Wilfried


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6117179 - 10/04/13 12:29 PM

Quote:

... So, you're using just enough magnification to suit your acuity?




Norme, basically yes but if seeing allows it I go a step higher in my eyepieces means for example x200 instead of x140 or x280 instead of x200 before I use my iris or the CO mask. Then I keep this magnification while doing my experiments to stay consistent in my setup.
Wilfried
PS: When using the iris I go sometimes down to 60mm aperture or even less with x280 and have still an image of reasonable quality despite magnification 5 times the aperture in mm - must have something to do with focal ratio (with 60mm ~f/16)


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Re: Next experiments with CO mask new [Re: WRAK]
      #6117237 - 10/04/13 12:54 PM

With my preliminary results I am so far less impressed with the hints for serious degradation of the image with larger CO values (may be I have a biased perception here as my C925 with 0.38 CO does despite a labor tested Strehl of 0.95 not perform as I hoped for) as with the obvious resolution enhancement of small to moderate CO values compared with zero CO. In most articles and books I read so far on the topic of resolving doubles the potential positive effect of reducing the size Airy disk by using reflectors with some CO compared to refractors is mentioned but considered as not significant enough to have any real effects for real world observing experience.
Now it seems evident that this may not be true and that small CO values bring a noticeable benefit for resolving of not only equal but to some degree also unequal doubles.
I have this effect already implemented in my RoT algorithm but was until now less than confident that this should be such a good idea - now I think this as confirmed by my observations. But certainly further experiments are needed, especially to find the turning delta_m point.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6140688 - 10/16/13 11:17 AM

Not many possibilities to continue with my experiments due to lack of clear skies.
Last night 52 Cyg/STF2726 6" +4.2/8.7mag was a nice target with large delta_m of 4.5mag.
Separation large enough to be easy resolved with apertures down to 80mm (may be even less with better seeing) - then I applied CO to 140mm.
CO 0.3 did not show significant changes in the image compared with zero CO, 0.35 made the companion slightly fuzzy and 0.4 made it hard to see the companion anymore.
I had the impression that in terms of resolution 80mm zero CO was equivalent to 140mm 0.4 CO.
Doing a simple calculation this would mean that a C9.25 with 0.38 CO is about equivalent with a 150mm refractor regarding resolution of heavy unequal binaries.
Now I run for cover.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6141137 - 10/16/13 03:18 PM

I found 52 Cyg almost a showcase double, very easy. 49 Cyg only slightly more difficult. Nailed STT 69 (1.5", 2.5 delta_M) a little easier than BU 67, but not BU 9AB (<1", 2.2 delta_M.) Seems once inside the first ring, lessor delta_M is needed - maybe 1.5 or less. Kui 97 was missed for the same reason.

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Re: Next experiments with CO mask new [Re: Asbytec]
      #6192121 - 11/13/13 07:29 AM

Last night half moon in the back, SQM ~17, NEML ~2.8, seeing Pickering ~6, diffraction pattern with some turbulences but crisp, halo Deneb ~60" gets completely dark when occulting Deneb (means good transparency).
Tried two equal doubles with 140mm APO with iris and CO mask:

STF2780 (HIP 104642) – 1" +6.09/6.77mag. Proposed aperture 128mm. With x140 for fractions of seconds split at 4 o'clock. x200 clean split. x280 with 93mm aperture elongation, 100mm rod, 105mm for fractions of seconds split. With 140mm and CO 0.2 split somewhat crisper than with zero CO. With CO 0.35 still clear split with hint of diffraction rings. With CO 0.4 diffraction rings overlapping central disks, but still clear split.
Conclusion: CO size is of minor relevance for splitting bright equal binaries.

Then STF2795 (HIP 105429) – 1.7" +9.31/9.66mag. Proposed aperture 109mm. x75 hint at 7 o'clock, x140 clean split for seconds. x200 limit aperture for split 95mm, down to 80mm rod. With 140mm and CO 0.2 split somewhat more stable than with zero CO, with CO 0.35 disks tad fuzzy. With CO 0.4 no longer split but rod.
Conclusion: CO value of relevance for resolving faint equal binaries - in this case 140mm aperture with CO 0.4 equivalent to 80mm aperture with zero CO.

In both cases limit aperture quite below RoT proposed aperture - means rather favorable observing conditions despite the half moon.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6192186 - 11/13/13 08:33 AM

Quote:

Conclusion: CO value of relevance for resolving faint equal binaries - in this case 140mm aperture with CO 0.4 equivalent to 80mm aperture with zero CO.




Interesting. Wilfried, when you say a rod split, "equivalent to an 80mm," I assume you mean it's a bump on the first bright ring and not a classic case of two visible discs touching. This 'bump' is often what I see with pairs (equal and unequal) with a separation slightly larger than the first ring radius.

The first ring (unobstructed) should be close to 1.3" arc and the companion just outside. The Airy disc inside that radius and the visible disc even inside the latter radius. So, there should be dark space between the discs. Are you reporting there is not, and that at 1.7" it is rod shaped?

It's possible. Astronomical light is more (but not entirely) incoherent and this does not allow FWHM to shrink significantly compared to coherent light. However, the Airy disc radius (all base PSF radii) do change by the same amount on both forms of light. But, again, FWHM is less affected in incoherent light. However, that you do see some adjustment at 0.2D suggests astronomical light does behave something like coherent light. My own split just below Dawes (as reported on 7 Tau) and the suggestion I could go deeper suggest the same (at least in diffraction limited seeing.)

What's interesting is your faint equal pair is not too dissimilar to BU 1030 (~0.8" arc). Yes, I got a (what appeared to be tiny) rod shape, too, often enough. But during calm moments, there was a distinct dark space.

I still do not know how to explain what you're seeing. Light is something between coherent and incoherent (no doubt measured in a lab under perfect 'diffraction limited' conditions and with computer modeling.) If seeing is diffraction limited, it's about as close to lab conditions as one can get in the real world. Truthfully, I have been amazed by what seeing can do when it's not disturbing the image. Io's 'apparent' elongation is one example, I doubt seriously it can readily be done in 6/10 seeing. It's just too messy.

Usually closer to 10th magnitude I am seeing pinpoints rather than full visible discs (and no ring structure.) I'm wondering in the 140mm if you are approaching some dim limit where the eye response begins to fall off making the visible disc smaller. (I'm looking for that effect, but pretty much can say even 7th mag stars nearly fill the dark space inside the first ring much like Sirius does. Still mulling that topic over.)

Edited by Asbytec (11/13/13 08:41 AM)


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6192354 - 11/13/13 10:13 AM

Wilfreid Im not holding with your finds that a 140 mm refractor with.4 CO equates to an 80mm unobstructed view at the stated magnitudes. An 80mm of any breed would perform miserably compared to an obstructed 140mm with a .4. There's s detrimental ingredient you are applying to COs that I don't find much agreement with.

Pete


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6192412 - 11/13/13 10:49 AM

Possible explanation: At +9.31/9.66mag I don't see any diffraction pattern with zero CO or small CO but only the central disk at least I assume - delta_m between central disk and first ring puts the first ring below my scopes magnitude limit.
With CO 0.4 things are different as the central disk gets fainter and first ring gets brighter resulting in a smaller delta_m between disk and first ring. May be then I can see the first diffraction ring but not resolved but in a blur with the central disk - this would then be an explanation for the rod impression as I get in this case two rather fat blurred disks touching each other.
With better seeing and higher magnification this might be resolved better but with the given observation conditions (although besides LP not this bad) and the magnification I used thats what I got.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6194054 - 11/14/13 07:19 AM

Yea, as I recall you're using minimum magnification for your studies, at least sometime you report 140x or so. Maybe that low the discs are visibly close enough to for a rod. It's still interesting that stars that dim appear small and might even appear closer together. Thinking back on BU 1030, that was the impression I got. Anyway, Fred often talks about the realm of dimmer stars being somehow different. Maybe that's what we're seeing.

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Re: Next experiments with CO mask new [Re: Asbytec]
      #6194347 - 11/14/13 11:13 AM

Norme,
Interesting comparisons - however the energy graphs posted here for increasing obstructions are seriously flawed. The 70-90% obstruction graphs show a central disk almost the same intensity as the first diffraction ring. This is clearly false.

Perhaps you would like to revisit the code that created these graphs?

Glenn


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6194453 - 11/14/13 12:10 PM

Glenn, I grabbed that from an earlier SA thread, don't know how it was created or what code was used. Still interested in the complex interplay between near monochromatic coherent and incoherent light with human physiology. It's far more complex that imagined.

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Re: Next experiments with CO mask new [Re: Asbytec]
      #6194483 - 11/14/13 12:25 PM Attachment (12 downloads)

I see. It could be that code was only looking at the diffraction rings - don't know.

Here is an animation of increasing obstruction. Even with an increasing obstruction the central spot will remain brighter than anywhere else. This graphic is normalized so it does not show the whole image getting dimmer as more light is cut out. Also it is displayed at gamma 2 so the rings appear brighter than they actually are. The actual eyeball response is very difficult to model. Not only are everyone eyes different, but the response in general cannot be described by a gamma only.

The animation does clearly show the central spot getting smaller with increasing obstruction as expected.

Glenn


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6194705 - 11/14/13 02:22 PM

I think Norme's graph refers to the distribution of peak intensity and is in this context correct. It is amazing how quick peak intensity of the central disk drops with increasing CO values. I am not sure about the weight of animations in terms of proof for anything but the visual impression of the animation for CO 0.8 looks very much like what I have seen when experimenting with extreme CO values. The same or even larger amount of energy in the first diffraction ring seems to give due to the larger surface of the ring compared to the central disk the impression of less brightness. This is one aspect I have so far not been able to check with my CO experiments: Which amount of CO is necessary to "erase" a fainter companion sitting on the first ring - this would then give an empirical evidence to the term of delta_m between central disk and first ring.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6195348 - 11/14/13 07:48 PM Attachment (8 downloads)

I think the original animated plot above is faulty. First it has a problem near zero, there is a singularity there in computations and maybe whoever created it did not deal with it well. Or maybe its a graphic of rings only? Not sure, but I would not base any conclusions based on that graphic.

Even with an 80% obstruction the central spot will be clearly brighter than the diffraction rings. See attached animation. It shows the airy profiles for a 150mm scope with varying obstructions in green light with the actual angular spread shown in arc-seconds.

I would not question your findings however. For a large obstruction seeing has to be very very good for it to be in any way beneficial in splitting a binary. This is a known but little discussed difficulty of large obstructions. When seeing is mediocre, turbulence will scramble all that extra energy in the rings along with the central spot to yield a mess.

Glenn


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6195409 - 11/14/13 08:21 PM

Quote:

For a large obstruction seeing has to be very very good for it to be in any way beneficial in splitting a binary. This is a known but little discussed difficulty of large obstructions. When seeing is mediocre, turbulence will scramble all that extra energy in the rings along with the central spot to yield a mess.




Intuitively, that makes sense as long as the ring structure is visible. Seeing here in the tropics is outstanding, enough so that the actual brightness of the second and third thinner rings can be seen at times. Other times, they are more disrupted by even the least disturbance, but even out to the fourth ring can be seen.

Interestingly, I feel reducing my own CO down to about 30% (from 38%), I lost the fifth ring all together on stars bright enough to show them and the fourth seems less bright. That was the extend of visible improvement, and maybe a little improvement on lunar diffraction effects.

Planetary /might/ be better, but it was not immediately obvious. Sketches seemed to improve over time, but that could be conditions or experience rather than CO improvement. Maybe that light loss in the fifth ring and the almost invisible 4th ring was redistributed to the central disc. Maybe the inner rings are unnoticeable dimmer, too.

The jury is still out on unequal doubles, and really on close doubles too. But some observations, particularly of 7 Tau suggest both a split /possibly/ below Dawes and the extent of the dark space seen (in excellent seeing) suggests a closer split is possible. Still need to do a sampling of splits just below Dawes down to about 0.7" arc to see if most can be split.

There is some psychological improvement as well knowing peak intensity moved very close to the standard 80%.

Wonderful animations, by the way. Now, how the eye responds to the logarithm of intensity is weird.

Edited by Asbytec (11/14/13 08:29 PM)


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6195457 - 11/14/13 08:41 PM

Glenn thanks for clarifying - your points are appreciated.

Wilfreid, perhaps then better seeing is needed to more accurately assess larger COs?

Interesting points made here.

Pete


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6195953 - 11/15/13 05:09 AM Attachment (8 downloads)

Glenn, your input is appreciated very much as new ideas stimulate the discussion but I think a "competition" with animations does not help much to answer the question of the effects of CO regarding resolution of unequal binaries.
Besides your animation shows near 20% intensity for the Airy disk with CO 0.8 and this is wrong at with this value it is already only ~10%, else green light is 510nm and not 550nm as indicated in the graph but 550nm is anyway the usually used value for average visual impression.

To get nearer the question of visual brightness of especially the first ring in terms of magnitudes I would be happy to find a paper discussing this topic with some scientific background. For example the information given on http://www.telescope-optics.net/obstruction.htm is quite comprehensive but the CO dependent energy distribution within the diffraction pattern does not help very much as it seems quite complicated to "translate" energy values in a spot (spurious disk) and in a ring into differences in magnitude.
Chis Lord's paper on resolving unequal binaries includes a table with delta_m values but I did not find any explanations how he got to this values and so far I was not able to confirm or falsify these numbers neither theoretical nor emperical.
For zero CO he gives a delta_m of 4.39 - does this mean that a companion with +6.39mag sitting on the first ring would disappear if the primary is +2mag bright?
I still have to wait for a good opportunity to get empirical evidence here - for my 140mm refractor I need a double ~1.3" with delta_m ~3 to have a good chance for resolving with zero CO and then I can apply different sizes of CO to see when and if the secondary disappears. Bad news here is that I did not find this many candidates with such parameters - currently I wait for an opportunity to observe STT457 in Cep with a delta_m of 2.16. Could get interesting - will the companion disappear between CO 0.4 and 0.5 or may be earlier or even not at all?
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6196461 - 11/15/13 12:13 PM

Wilfried,

I'm not competing with any animations. I was simply correcting the original posted animation as it was incorrect.

I'm confident the last posted animation is correct. Photometric peak searches on high definition simulated star images yield a ratio for an obstruction of 0.8 as 0.1294 with respect to the unobstructed case. The Bessel expression used in my last posted animation shows it as 0.1296. That's close enough I think.

Glenn


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6196527 - 11/15/13 12:56 PM

Norme,

The diffraction rings do not need to be 'visible' to detract from an obstructed scopes performance in dodgy seeing. In fact in larger scopes and 'continental' seeing the diffraction rings are rarely visible. However the energy is still there and given turbulence impacts the scopes performance in a negative way in terms of contrast transfer. Think of an obstruction as a high pass filter, and when increased displays increasingly more noise.

I envy the tropical seeing you enjoy! I'm lucky to see fragments of the first diffraction ring in my C14.

Finally, you stated earlier "As I understand it, light is pretty much coherent in modest amateur scopes.". What evidence or reference do you have for this? I have never seen an image of a binary star that would suggest coherence, from any scope. But if you have any evidence I would sorely like to see it.

Glenn


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6196756 - 11/15/13 03:53 PM

Well coherence here I think is being used in the way of stating medium apertures will more often yield a stable diffraction pattern
Than a larger aperture which can be quite a bit more demanding. Not to suggest large aperture is taking a back seat to resolution just that its more condition needy to get the same apparent stability in the pattern for average seeing conditions.

Pete


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6197235 - 11/15/13 09:25 PM

Glenn, yes, that's the interesting thing about doubles for me. The rings do not have to be visible to affect contrast transfer, so it seems to imply energy is affecting the companion in some way. This was kind of the core of my question earlier. Is it energy or glow from the brighter primary.

I can't offer proof for stellar light being coherent, in fact it's not. But, it's not entirely incoherent either, in how it behaves. For example, incoherent light (according to discussion here and here) really does not loose peak intensity in the presence of a CO as coherent does. So, that we do notice some intensity loss suggests a bit of coherent behavior. And FWHM is relatively unaffected for incoherent light, however if FWHM is affected with an obstruction then it would behave like coherent light even though it is nearly incoherent. And all of this behavior is overlaid by the human logarithmic response to intensity distribution. Complicated, in that it's nearly monochromatic. I am still trying to get my head around it.

"While there is no difference in a single point-source imaging between coherent and incoherent light with respect to the relative intensity distribution - as long as light remains near monochromatic - the resolution limit for a pair of point sources for the former varies with the phase difference between the two sources...Since, according to Van Cittert-Zernike Theorem, light arriving from stars is coherent in amateur-size telescopes, as long as it is near monochromatic, it is an interesting question how much this coherence factor, changing with the wavelength bandwidth and source OPD, influences the actual resolution limit in the field."

"Since deducting the CO area amplitude contribution and squaring the difference (in coherent light) produces different result than deducting energy from this area directly (in incoherent light), the entire annular aperture PSF will differ for the two, not only its central intensity. Considering that PSF-defined intensity distribution for point-source image determines contrast and resolution of extended objects as well, any significant difference in the annular PSF will imply significant difference in the effect of CO on overall performance."

My question is, "In general, CO effect modeled for incoherent light shows much less depressed central maxima, and nearly identical reduction in its width." Is this what we actually see? We obviously get to 80% intensity with a ~0.3D obstruction, so are we seeing 'behavior' consistent with coherent light for near monochromatic incoherent star light? And as such, does FWHM behave consistent with coherent light (affecting resolution), as well, by falling off more rapidly than with pure incoherent light?

"The actual light from astronomical objects is neither incoherent, nor coherent. It is generally closer to incoherent (partially incoherent), so the actual CO effect would be, correspondingly, closer to that indicated by the incoherent model. While this modeling for the effect of central obstruction in incoherent light is, as already mentioned, informal and approximate, it can be assumed that it is significantly smaller than in coherent light, simply because taking out energy originating from the obstruction area directly has significantly less of an effect than taking out the corresponding wave amplitude in coherent light."

Much of this technical discussion is over my head and I'm swimming for the surface to breath.


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6197510 - 11/16/13 12:32 AM

Norme,
We're straying off the OP comments - not your fault really - I'll share some of the blame!

I respect Vlads contributions in his website you linked to. I would love to see him publish his work in printed form some day as many of his graphics and discussions are unique and of much value to astronomers interested in telescope performance.

Coherent optics forms the basis for incoherent imaging. From the coherent model one can build a structure of expressions using sums of uncorrelated waves which describe very well the way images are created.

My question was with respect to a binary system. There is no coherence between 2 or more stars closely placed - their wavefronts are independent from each other. So there is no way the sum of the stars would yield a coherent interference pattern. If there was coherence the 2 stars could show alternately a bright max or a zero min or no difference at all depending on the phase difference between them.

In a microscope however, especially when its condensor iris is stopped down or when using laser illumination, coherent images are clearly evident. I've done a lot of work in that area, and in fact most of my code was written for coherent imaging. I know what it looks like and I have never seen myself or seen published any evidence of coherence for astronomical images in optical wavelengths.

When reading texts on optics its important to understand that some limitations discussed (like the utility of a MTF for example) apply to one or the other as they are meant to describe imaging in general - like everything from microscopes to telescopes as I mentioned.

But to answer your question - the PSF in my graphics above are what we would see for a coherent point source imaged in a narrow wavelength range. Full understanding requires reading Vlad's work in addition to several other texts and especially the limitations of the Van Cittert - Zernike theorem. For visual work the image should resemble the incoherent expectation, so the central max would not be diminished as much with increasing obstruction. Need to think about this one, I see your dilemma!

Glenn


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6197585 - 11/16/13 02:16 AM

Thanks, Glenn...yea, I am a long way from anything beyond a faint understanding of how it all comes together. Just delving into the complex topic. Great point on the lack of coherence between two closely spaced stars, but the individual star has an essentially 'coherent' point source wavefront?

Yea, we may be off on a tangent, but I thought it might be applicable to discussing CO and particularly on doubles...and related to the topic. If it's not, forward any Moddie PMs to me, I'll apologize profusely, eat crow, and take the hit to my reputation for dragging the discussion here.

So, yea, why do we get less peak intensity if light is more incoherent than not. Actually, in good seeing I've peaked deep into the Airy disc realm and feel I can best Dawes with an obstruction. It's given me a new appreciation for that bit of high performance curve near max spacial frequency (and the PSF.) So, not sure what's going on down there.

Cheers, Glenn

Edited by Asbytec (11/16/13 03:46 AM)


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6197648 - 11/16/13 05:38 AM

Quote:

the individual star has an essentially 'coherent' point source wavefront?



No, the stellar photosphere consists of a vast number of independent "transmitters"; putting enough distance between the stellar surface and the photon collector / detector to make the star appear as a "point" (angular diameter much smaller than the instrumental diffraction pattern) doesn't affect coherence.

What does make a difference - in an incoherent stream - is a tight waveband limit. The point here is that there is no difference in atmospheric disturbance caused by variations in refraction with wavelength so the image appears sharper. Most "white light" solar observers are well aware of the improvement in seeing when a narrow waveband (nebula type) filter is used but it appears that few deep sky observers know about this.

Quote:

Yea, we may be off on a tangent, but I thought it might be applicable to discussing CO and particularly on doubles...and related to the topic. If it's not, forward any Moddie PMs to me, I'll apologize profusely, eat crow, and take the hit to my reputation for dragging the discussion here.



No need IMVHO. Interesting discussion often arises from going off at a tangent, but this is related rather than tangential.

FWIW my personal opinion is that low CO systems seem to suffer less from atmospheric turbulence than high CO systems of a similar aperture / resolving power / light gathering capacity. Theories about thermal disturbance of the air around the secondary don't seem to hold water as (a) thermal disturbance from the objective, which has much more thermal inertia, will completely mask them, and (b) the practical seeing advantage of a small CO persists when the scope is fully conformant with ambient temperature.

What really matters here is the stability (lack of turbulence) of the atmosphere for the whole light path from the top of the atmosphere to the light detector (be it animal or mineral) via the objective. The diffraction pattern is indeed modified by the central obstruction but the practical effects of this are usually quite small for COs of up to 30 - 35% providing that the seeing is fairly steady.


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Re: Next experiments with CO mask new [Re: brianb11213]
      #6197701 - 11/16/13 07:38 AM

I think I have meanwhile found how Lord calculated his delta_m numbers for the diffraction pattern: Translating the relation of peak intensity into magnitude differences.
Examples:
0.0 CO: Disk Peak=1, First Ring Peak=0.0175, Relation ~57, Delta_m based on LOG2.512 ~4.39
0.1 CO: Disk Peak=0.98, First Ring Peak=0.0206, Relation ~47.5, Delta_m based on LOG2.512 ~4.19
0.2 CO: Disk Peak=0.92, First Ring Peak=0.0304, Relation ~30, Delta_m based on LOG2.512 ~3.70
0.3 CO: Disk Peak=0.83, First Ring Peak=0.0475, Relation ~17.5, Delta_m based on LOG2.512 ~3.11
0.4 CO: Disk Peak=0.71, First Ring Peak=0.0707, Relation ~10, Delta_m based on LOG2.512 ~2.5.
The numbers do not meet exactly the numbers of Lord but to a good enough degree to assume that his reasoning was along this line.
While I don't want to suggest that these numbers are "true" (especially not without empirical evidence) the idea to use peak intensity numbers for calculating delta_m values makes sense for me as this would it make possible to directly compare the peak intensity of the first ring of the primary with the peak intensity of the central disk of the secondary.

I still hope that somebody with better knowledge of diffraction theory and better mathematical background than me comes up with a solid founded approach to determine delta_m within the diffraction pattern.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6197788 - 11/16/13 09:03 AM

I think it's a great discussion and I appreciate everyone who contributes for contributing.

Wilfried, I still find your calculations above interesting. At 0.3D, I look for those unequal doubles in the range of deltaM ~3 in the vicinity of the first ring. Some closer and some wider, some dimmer and some brighter. I think there is something to the figure calculated (more or less) at the angular separation of the first ring.


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Re: Next experiments with CO mask new [Re: brianb11213]
      #6197972 - 11/16/13 11:25 AM

Brian,

If I understand what Norme is asking we have this. The experimental evidence of CO effect on the PSF implies the distribution is more like the expected coherent behavior. At least across the distance from the central max to the first minimum - or possibly even a lesser distance. To answer that requires some computations using the Van Cittert Zernike theorem.

If the star was really incoherent the drop in PSF max due to obstruction is entirely different and much smaller. Thus the question Norme is asking.

Complicating matters is this: the intensity of the central max is highly dependent on wavelength. The power goes as the inverse square of wavelength. So the central max in blue light would be significantly brighter than red, taking into account the spectral distribution of the star.

On seeing - yes a CO makes seeing more of a limiting factor, for the reasons I stated above. Even if the outer rings are not visible, common with larger scopes, there is enough energy there that in the presence of turbulence the image is more negatively impacted with increasing CO.

Glenn


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6198155 - 11/16/13 01:16 PM

Yes, that's exactly the question, the apparent coherent behavior...if indeed it is apparent rather than normal incoherent behavior. Or something in between.

On seeing. Seems that tropical excelent seeing has finally arrived, even with a CO. Sirius just sat there as a disc and rings - sometime a bit dusturbed. Amazing sight, just wanted to share the moment. No confirmed pup sighting, though. Straying back toward the topic, "darn CO!"

Edited by Asbytec (11/16/13 01:17 PM)


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6198218 - 11/16/13 01:49 PM

Checked my data set and found several observations from Fred, Norme, Roberto and myself with delta_m ~3 at or near the first diffraction ring. I found also an entry from Roberto for AC15 1.25" +5.13/8.96mag meaning delta_m of 3.8 directly on the first diffraction ring (with 152mm and zero CO within the calculated numbers).
Only one entry in my data set is outside the above calculated values: Bill's observation of Gam Equ with CO near 0.3 but delta_m of 4 - but this is a case with open questions regarding the advertised data.
Wilfried


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Re: Next experiments with CO mask new [Re: Asbytec]
      #6198320 - 11/16/13 03:10 PM

Quote:

Sirius just sat there as a disc and rings - sometime a bit dusturbed. Amazing sight, just wanted to share the moment.



Now you are just making us envious! I use to live in a semi-tropical clime and the seeing was clearly much better than here in the desert southwest. Your statement above just hurts!

On coherence
Just to be clear to anyone reading here - we are discussing the concept of spatial coherence. If one were to place 2 pinholes or slits in the intercepted star beam (Young's experiment) the contrast in the resulting inteference pattern beyond would be a measure of the degree of spatial coherence. Best done with color filter. I suspect it is fairly coherent in that respect. One could get a handle on it, if not measure, by estimating the contrast in the bands seen with a ronchi eyepiece.

I think it fairly safe to model these PSF's as above - which assumed coherent, single wavelength light. Doing the incoherent pattern is relatively easy, partial coherence somewhat more difficult.

Glenn


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Re: Next experiments with CO mask new [Re: DesertRat]
      #6198441 - 11/16/13 04:28 PM

To make things a bit more realistic we should may be include the factor quality of scope in terms of Strehl.
Formula for calculating max. CO for target peak intensity depending on Strehl:
max. CO =(0.6-0.6*Peak Intensity/Strehl)^0.5.
Example for target peak intensity 0.85 with given Strehl 0.95 (not perfect but very nice scope) results in max. CO = 0.25.
Example for "target" (nobody would want this) peak intensity of 0.1 with given Strehl 0.95 results in max. CO = 0.73.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6198905 - 11/16/13 08:52 PM

Glenn, my apologies. Last thing I want is to inflict pain, well most of the time, anyway. I lived 100 miles north with exceptional nightly seeing, this new location had yet to prove itself. I've been very fearful about moving away being a horrible mistake. So, to see a really good night was a relief of mammoth proportions. Being retired, seeing alone is enough to make me move back. So, yea, it was great to finally 'see' it after fretting over something so important to us.

To be fair, I am equally envious of your (Brian, et al) grasp of the topic at hand. It's almost painful, too, to be so behind in the knowledge with a long hard road ahead.

On coherence as it relates to doubles, it does seem each star has many emitters therefore the light arrives not in phase. It's not a laser. But the idea of spacial coherence is interesting, being two individual points close in angular separation and possibly physically close on astronomical scales.

The distance between them, of course, dwarfs the wavelength of light, but can they somehow be coherent in that the wavefront might arrive almost as if from a single source? The two slit example you mention is the same wavefront arriving the detector and seems not to dissimilar to two closely spaced sources whose wavefronts experience very similar seeing and aberration en route to the focal plane.

Can the wavefront leaving one source arrive in phase and on time with a wavefront from the companion source? Is that a form of coherence? It seems the same could be said for many emitters on a single star. Some waves from different emitters are bound to arrive in phase (for a given color.)

Edited by Asbytec (11/16/13 10:02 PM)


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Re: Next experiments with CO mask new [Re: WRAK]
      #6198909 - 11/16/13 08:55 PM

Quote:

Checked my data set and found several observations from Fred, Norme, Roberto and myself with delta_m ~3 at or near the first diffraction ring. I found also an entry from Roberto for AC15 1.25" +5.13/8.96mag meaning delta_m of 3.8 directly on the first diffraction ring (with 152mm and zero CO within the calculated numbers).
Only one entry in my data set is outside the above calculated values: Bill's observation of Gam Equ with CO near 0.3 but delta_m of 4 - but this is a case with open questions regarding the advertised data.
Wilfried




Wilfried, looking through my observation files of difficult pairs, from the 140mm refractor, I'm finding various doubles on or near the first bright ring with a delta-m in the 2.5-3 range. The same delta-m range applies for the more difficult pairs that fall into the Rayleigh gap - the first dark interspace, closer to the primary's disc. I find it interesting that there's an approximate matching between a closer companion in a dark space, and a slightly wider one on a bright ring, in terms of delta-m.

Wider again, beyond the first bright ring, around the 1.7" to 2.0" separation, several pairs with delta-m in the 3.5 to 4 range were seen. (For 140mm, the 2nd bright ring is centred about 2.2"). So, clear of the first ring, and slightly wider, delta-m can unsurprisingly be larger and still allow a visible star.

On the matter of Gamma Equ, we're still in the dark so to speak, until there's a new measure.

With regard to another of the doubles on the Haas list, 42 Ori, I've been looking at the complete data file, where before I had only some of the measures, and will shortly (later today?) offer a revised commentary on that one. 42 Ori has had various discussions in a few threads here in the past.

I did a revision recently on BU 9 (in the "From Dolphins to Dogs" thread) where I discovered that having the complete data set changed to some degree the overall picture, compared with what could be deduced from a partial data set. Likewise with 42 Ori, the full data set suggests a different pattern.


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Re: Next experiments with CO mask new [Re: fred1871]
      #6198965 - 11/16/13 09:36 PM

Fred, with my own limited double experience, I find the delta mag on or inside the first ring to require a brighter companion than say about mag 3, too. I don't know the delta M required, it might even change nightly. But it seems delta mag 3 is a bit optimistic if not a close approximation. It is interesting this delta mag limit seems to hold into the first dark inter-space and possibly decreasing closer to the primary. It's one of the fascinating mysteries of doubles. What makes the so difficult? Surely glare from the primary, diffraction plays a role somehow, as does personal acuity and our physiological response along with observing conditions and optical performance.

BU 1030 is a real mystery in that each visible disc should retain much of it's angular size even being fairly dim. In appearance, however, the pair appeared to be two very closely separated pin points. Surely each star affects the other in many ways with a delta mag of ~1.9. That they 'appeared' to be very close pin points, both neatly tucked inside a faint ring structure, with a dark space was astonishing, really.

On 42 Ori, that was one of those long duration observations that finally led to the correct 'guess' to the companion's location. There was discussion about how conditions might sweep the first ring long enough to get a good look at the companion (hampered by the CO effect on the first ring and maybe reduced intensity of the central disc.) I am not sure, but then sometimes it seems the only explanation, this allowed a lucky guess to be correct. It's becoming clear, to me, steady ring structure in good seeing is preferable for doubles. Man, fighting that distorted ring and all the seeing artifacts simply makes resolution a real chore on difficult doubles, so not sure the 'ring sweeping' effect is preferable.


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Re: Next experiments with CO mask new [Re: WRAK]
      #6199066 - 11/16/13 10:42 PM

Quote:

I think I have meanwhile found how Lord calculated his delta_m numbers for the diffraction pattern: Translating the relation of peak intensity into magnitude differences.
Examples:
0.0 CO: Disk Peak=1, First Ring Peak=0.0175, Relation ~57, Delta_m based on LOG2.512 ~4.39
0.1 CO: Disk Peak=0.98, First Ring Peak=0.0206, Relation ~47.5, Delta_m based on LOG2.512 ~4.19
0.2 CO: Disk Peak=0.92, First Ring Peak=0.0304, Relation ~30, Delta_m based on LOG2.512 ~3.70
0.3 CO: Disk Peak=0.83, First Ring Peak=0.0475, Relation ~17.5, Delta_m based on LOG2.512 ~3.11
0.4 CO: Disk Peak=0.71, First Ring Peak=0.0707, Relation ~10, Delta_m based on LOG2.512 ~2.5.
The numbers do not meet exactly the numbers of Lord but to a good enough degree to assume that his reasoning was along this line.
While I don't want to suggest that these numbers are "true" (especially not without empirical evidence) the idea to use peak intensity numbers for calculating delta_m values makes sense for me as this would it make possible to directly compare the peak intensity of the first ring of the primary with the peak intensity of the central disk of the secondary.
.......
Wilfried




Wilfried, nice to have confirmation of Lord's delta-m numbers ... data for doing this calculation, though only for unobstructed apertures, is as I mentioned recently, available in Thomas Lewis's 1914 paper. There he gives (The Observatory, October 1914, p479) a table of "relative illumination" at various radii from the centre of the diffraction image of a star, and these numbers, as he says there, can be used to derive "the corresponding star magnitude".

At the first ring maximum, Lewis gives a relative illumination of 17 compared to 1000, the latter normalised for the centre of the diffraction disc (.017/1.000). That corresponds to a ratio of just under 60, which on the log magnitude scale is ~4.4 magnitudes - that gives a delta-m.

The second ring maximum (unobstructed) is only about 4 units, or not quite 1/4 of the first ring, so a difference near 4:1, therefore a further ~1.5 mags fainter - delta-m near 5.9. And so on.

For obstructed apertures the corresponding illumination levels for the centre of each ring could be calculated and used.

One interesting feature is that the rings are of some width and have varying illumination across their width, being brightest at the centre of course. For a 140mm refractor, relative illumination in the first ring changes little over about the 1.2" to 1.4" range (centre at ~1.3"); moving outwards, the illumination of the ring as we approach 1.6" has dropped to about 1/3 of what it was centrally; so delta-m is now around 5.6; from there it falls off sharply, being at or near zero from 1.7" to 1.9".

Evaluating observations will, I think, be helped by taking into account that the rings show some width. This will be significant mainly with the first ring, being much the brightest; so that a star does not have to be exactly on the ring centre to be affected. My previous note said that I'm not finding a noticeable difference between "in the first space" and "on the first ring". If that proves correct, the obvious visibility change will occur when a secondary star is that little bit wider than the extended bright area of the first ring, where I've found stars being visible with larger delta-m.

Further observational data will be needed to see what difference is made by the second ring (unobstructed apertures) and the third (obstructed apertures).


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Re: Next experiments with CO mask new [Re: fred1871]
      #6199211 - 11/17/13 12:13 AM

I have seen where a ring swept momentarily out of the way helped to clarify things though. Maybe if I had more nights like you enjoy in your observing season Id think different. Lambda Cygni comes to mind where the rings when blown aside actually helped some. I'm thinking back when it was 0.7" not the current 0.9". This IS a good double by the way.



Pete

Edited by azure1961p (11/17/13 12:16 AM)


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6199394 - 11/17/13 04:47 AM

The different surface size of central disk and diffraction rings combined with the energy distribution in the pattern were initially confusing aspects for me but peak intensity made things somewhat more clear as peak intensity does dot depend on surface. But as there is a distribution of brightness over the surface there should be still some effect on resolving fainter companions even when not directly sitting on the center of the ring as Fred mentions.

If the above calculated numbers are near reality then theoretically this would mean now hat we should be able to resolve with zero CO a companion somewhat brighter than ~4.4mag delta_m to the primary at or near the first ring. Empirical observations results show that this is obviously the case only for secondaries at least one mag brighter than the ring - this might mean that the peak brightness of the first ring is not the limiting factor here but other factors or it may also mean that this is very well the case but that the human eye needs at least this difference in magnitude to resolve such a situation. My planned experiment with different CO values and therefore different brightness of the first ring for doubles with a companion on the first ring should answer this question.

Back to the above mentioned formula with peak intensity, Strehl and CO - this rings in my memory the topic of diffraction limited resolution according to Rayleigh. The above mentioned combination of CO 0.25 with Strehl 0.95 giving a peak intensity of 0.85 would then be a perfect match with my observation results with different sizes of CO and thus a scientific reasoning for the degradation of the image with CO values larger than 0.25 assuming a scope with Strehl 9.95. You would then need a perfect scope with Strehl 0.99 to get a peak intensity of 85% with CO 0.3 - thus CO 0.3 would then be the upper CO limit for resolution near the theoretical optimum even with a perfect scope.
Wilfried


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Re: Next experiments with CO mask new [Re: WRAK]
      #6199396 - 11/17/13 04:51 AM

Quote:

I still hope that somebody with better knowledge of diffraction theory and better mathematical background than me comes up with a solid founded approach to determine delta_m within the diffraction pattern.
Wilfried



The maths is actually rather trivial, the issues here are that variability in human sight, in particular the skill to pick out small scale low contrast features in a target object continuously subject to atmospheric variations, are such that a mathematical model is not going to give much more than a hint of what is observable under practical observational conditions.

If you make assumptions about seeing being "perfect" then experiments based on calculated diffraction patterns can be arranged by e.g. printing the diffraction image of a primary star and its companion onto high resolution copy film, illuminating from behind with a light box and observing using a loupe (magnifying glass). The diffraction pattern can be calculated for varying central obstructions, blurred to allow for optical imperfections, illumination can be changed to allow for variations in brightness, all without having to wait for a clear night and with guaranteed perfect seeing.

But the really interesting result from such an experiment would be to compare it with the results of many decades of practical observations obtained through the earth's perpetually turbulent atmosphere.


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Re: Next experiments with CO mask new [Re: WRAK]
      #6199640 - 11/17/13 10:33 AM

Norme,
There is zero correlation between 2 independent stars. And even if there were seeing effects would effectively destroy it.

Wilfried,
Yes more realistic simulations are possible. Not only aberrations but seeing can also be simulated - though that is a more difficult problem as seeing has so many free parameters. Some seeing regimes are almost diabolical, and whats worse are seeing regimes which have non-random characteristics.

Note also that not all aberrations are equal. Although it is true that small rms aberrations terms contribute the same effective Strehl reductions - it is not true they are all the same in their impact to splitting binaries. For example you could have a small angular dependent aberration (coma, astigmatism, etc) and still split or not split a binary depending on the relative geometry of the binary. This is related to the known limitation of evaluating an optic based on its 'splitting' capability.

As useful as simulations can be, experimental results are more useful in my opinion. I think enough is known about the theoretical constraints of obstruction that your investigation holds more promise to identify and/or confirm the expectation of its effects.

Glenn


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Re: Next experiments with CO mask new [Re: brianb11213]
      #6199867 - 11/17/13 12:38 PM

Quote:

Quote:

I still hope that somebody with better knowledge of diffraction theory and better mathematical background than me comes up with a solid founded approach to determine delta_m within the diffraction pattern.
Wilfried



The maths is actually rather trivial, the issues here are that variability in human sight, in particular the skill to pick out small scale low contrast features in a target object continuously subject to atmospheric variations, are such that a mathematical model is not going to give much more than a hint of what is observable under practical observational conditions.

If you make assumptions about seeing being "perfect" then experiments based on calculated diffraction patterns can be arranged by e.g. printing the diffraction image of a primary star and its companion onto high resolution copy film, illuminating from behind with a light box and observing using a loupe (magnifying glass). The diffraction pattern can be calculated for varying central obstructions, blurred to allow for optical imperfections, illumination can be changed to allow for variations in brightness, all without having to wait for a clear night and with guaranteed perfect seeing.

But the really interesting result from such an experiment would be to compare it with the results of many decades of practical observations obtained through the earth's perpetually turbulent atmosphere.




Exactly Brian. I've found a lot of discord between the CO experiments posted in this forum and my own finds to the point that its more indicative of one persons skills without the benefit of an emperical benchmark that holds value elsewhere. Quantifying and averaging out the decades of work as you mention or the past centuries would be more revealing than what might be found here. I'm only one person too but I can not say Ive found the CO results reflective of anything Ive seen first hand. I think this seeking of a RoT is fine for the author of it but I can't see extending it to other people as any kind of baseline standard for expectations - particularly if it falls short repeatedly of what others might be seeing.

The RoT for me is something to be extracted from history of others with less reliance in personal experiences .

Pete


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Re: Next experiments with CO mask new [Re: brianb11213]
      #6199976 - 11/17/13 01:44 PM

Quote:

... The maths is actually rather trivial...




Brian, great news. I am so far without result on the lookout for a spreadsheet implementation of the diffraction pattern algorithm giving angular size of at least 5-6 maxima/minima for given aperture and CO and the CO depending peak intesities for the maxima. Should be possible as the Bessel functions are implemented in Excel but this will still need some programming above my skill level. May be you can help me here.
And despite the opinion that everything about the effects of CO on resolving unequal doubles is known I am still without success on the lookout for an article with scientific background about the positioning of the central disk of the secondary in the diffraction pattern of the primary on base of peak intensities. May be this is also a point where you can help me.
Wilfried
PS: The idea to sample historic observation reports for getting raw data for developping a RoT algorithm is doomed from the very beginning:
- Nearly all reported observations are usually done with a fixed aperture and CO combination and are therefore on the limit only by a very small chance. This is the reason why I do my observations with variable aperture and variable CO size
- The quality of the advertised data for doubles is often troublesome today and even the more in the past. This is the reason why I counter check routinely the WDS advertised data of doubles I observe with APASS and UCAC4 and in future also with USNO A2


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Re: Next experiments with CO mask new [Re: azure1961p]
      #6200556 - 11/17/13 07:23 PM

Pete, Wilfried's comments point to the kinds of problems with harvesting material from the history of double star observing. Apart from most of it not being "limit observations", the continuing issue of getting accurate photometry is a major issue. Some time ago I suggested getting data from historical records of double stars, but suggested that it would require re-assessment compared with, for example, Thomas Lewis's study of this type in 1914. At the time Lewis did this the photometric accuracy of his data was not very good.

An example - if you look at Lewis's table of results, listed by observer and telescope, you'll notice that George Knott did better than most in the unequal pairs, 3m difference, category - but that's in part due to a selection effect on stars with poor photometry, such as the (now dreaded) Gamma Equ, originally estimated to have a mag 11 companion. The "photometry effect" is a biggie back then.

Separation measures, less problematic. Though these days the accuracy of separation measures using speckle or adaptive optics techniques on large telescopes are more consistent and accurate than was possible a hundred or more years ago.

But there's no reason to think our current efforts are doomed to be merely personal results. Wilfried's approach, using varying apertures and CO, seems to me very useful and likely to give good results when there's a large enough collection of data points. And some of the results obtained thus far are consistent with limit observations obtained in the past, if we check out those past observations using the best photometry and measures we now have.

Could you tell us more of what you mean in the comment "I can not say Ive found the CO results reflective of anything Ive seen first hand"? What are you seeing that's different? - because it could well be useful in data terms, or in suggesting possibilities overlooked so far.

I will say that "personal experiences" are not a uniform category - some are unrepeatable, because they were false positives, which will happen to observers from time to time; or were remarkable "one-off" moments; others are repeatable by other observers with the same telescope size etc, so they're useful data points. Repeatable is what this is about.

There will always be a sort of fuzzy edge to what a particular observer can achieve. And some will go a little further, some a little less far, than than the majority. But, given the limits set by physical optics, the variations of observers will tend to average out in a particular small region - so we might be able, eventually, to say - with an unobstructed telescope of 150mm aperture, a double star of delta-m 3 with the secondary star brighter than mag 10 can be seen at the Rayleigh criterion separation by an experienced observer in conditions of low turbulence, with good probability. Not nearly as neat as Rayleigh or Dawes; but it's likely similar to what we'll get to eventually, though with an algorithm that covers multiple possibilities, of which that is one example.


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Re: Next experiments with CO mask new [Re: fred1871]
      #6200999 - 11/18/13 01:43 AM

Hi Fred,

I appreciate your thoughts here. I also believe in the arriving at an RoT - where Ive found issue with Wilfrieds assessments here are with the detrimental effects of CO . I agree with him that an ever larger CO can blot out a fainter companion that's too near it covered by the brighter large CO diffraction ring(s). Its the degree with which his finds conclude that a .40 CO is significantly detracting from resolution - one example comparing it to a small 80mm refractor . There have been other examples that just don't agree with my finds and Im left feeling often enough the figures don't represent what Im experiencing.

I understand the points you raise in the difficulties of history providing the answers.

Pete

Edited by azure1961p (11/18/13 01:44 AM)


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Preliminary results and still open questions new [Re: azure1961p]
      #6201598 - 11/18/13 12:25 PM

The so far preliminary results for me are:
- Small CO values between 0.15 to 0.20 enhance resolution compared to zero CO. This empirical impression is backed by diffraction theory due to the calculated CO related reduction of Airy disk size even if it seems rather counter intuitive for a refractor user - the idea of enhancing the resolution by putting a central obstruction before or on the lense hurts a bit
- A CO size of 0.25 to 0.30 gives resolution comparable to zero CO for equal binaries and unequal binaries up to some delta_m. This empirical impression is backed by theoretical considerations regarding diffraction limited resolution (based on the work of Rayleigh and others) requiring 85% peak intensity for results better than 1/4 wave deviation. And this is for CO 0.25 to 0.30 possible only for very good scopes with Strehl 0.95-0.99 and no longer possible even for perfect optics with CO larger 0.3
- A CO size larger 0.30 leads to increasing degradation of resolution especially for unequal doubles. This empirical impression is backed by the same diffration limited resolution considerations as mentioned above
- A concept for calculating the relative brightness of diffraction rings was found on the base of peak intensities which is so far weakly supported by empirical impressions.

Still open questions:
- Delta_m limits for enhanced resolution with small CO sizes
- Delta_m limits for zero CO like resolution for CO 0.25 to 0.30
- Empirical correlation of peak intensity degradation by increased sizes of CO with degradation of resolution
- Empirical support for the assumption that companions with given faintness can be "erased" with increasing brightness of the first diffraction ring by increasing CO sizes
- Empirical check of the validity for calculation of ring brightness on base of peak intensity by comparing the visibility of rings of faint primaries with the current telescope magnitude limit
- Empirical check of the required delta_m for resolving faint companions against the brightness of diffraction rings.

Some more questions will arise with further experiments and hopefully emerging further theoretical considerations.
Wilfried


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Re: Preliminary results and still open questions new [Re: WRAK]
      #6208924 - 11/22/13 08:46 AM

Ongoing fog and clouds disable observations but enable rereading interesting articles on the topic of double star resolution with meanwhile gained additional intelligence. So I stumbled again over Herschel's recommendation to put a central obstruction "centrally before the object-glass, having a diamenter from a sixth to a fifth of that of the object-glass" to enhance double star resolution. So far I understood this as vague proposition but now I see that this fits exactly the CO size range I found by experiment actually enhancing resolution.

Foul weather also gives me time to develop a solution for a combined iris and CO mask. With a fixed aperture CO mask it is very difficult to find pairs suited to check the brightness of the first diffraction ring against the brightness of the companion (as it should sit in the ring and have a delta_m of ~3mag) but if I can also change the aperture then the number of suited pairs is much larger as I am no longer restricted to a specific separation but only to a separation range.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6222674 - 11/29/13 01:11 AM

have any of you fellows ever tried Cardboard Double-Star Interferometer..
mike hyrczyk


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Re: How good are reflectors for resolving binaries? new [Re: Perseus_m45]
      #6222780 - 11/29/13 03:36 AM

Have read about it and made a mental note to try it eventually if I ever make the step to measuring separation of doubles with my C9.25.
Have tried to find the old article from Maurer in Sky&Telescope 1997 how to make one but did not succeed.
Wilfried


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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6222820 - 11/29/13 04:57 AM

I might be interested, too, if I could find how to do it.

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Re: How good are reflectors for resolving binaries? new [Re: Asbytec]
      #6222840 - 11/29/13 05:18 AM

I think this might be the trick..
mike h

ftp://75.76.20.42/Shares/SkyAndTelescope/SkyandTelescope_1997%20-%20astronomy...


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Re: How good are reflectors for resolving binaries? new [Re: Perseus_m45]
      #6222852 - 11/29/13 05:39 AM

Mike, thanks, that look doable.

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First experiments with combined iris and CO mask new [Re: Asbytec]
      #6230871 - 12/03/13 07:29 AM

The number of clear nights is rather limited for me in this time of the year but I had during the last weeks twice the opportunity to do some experiments with the combined mask giving me greater flexibility in aperture and CO size combinations.
Last opportunity was last night with STF194 (HIP 9283) in Ari with 1.3“ +7.62/9.46mag. Started with 140mm and CO 0.15 and could easily resolve STF194 with a magnification of x200. Then to my surprise I was able to reduce aperture down to 100 mm (CO then 0.21) with still a clear resolution - my RoT suggests here 135mm. To my regret I was not quick-witted enough to instantly try again with zero CO as obviously seeing was much better than expected (thin cirrus clouds were present but the air was rather steady) but had then to quit this session as the clouds got more solid.
A few nights ago with somewhat not this good seeing I got for STF194 a rather fuzzy split with 140mm and CO 0.2 and had to note this aperture as limit.

Under the same not this good conditions I observed STF226 (HIP 10272) – 1.8" +8.02/9.64mag, resolved it with 140mm zero CO and magnification of x140. Could then with x280 reduce aperture to 100mm. With 140mm and CO 0.2 resolution was as expected a tad crisper but when reducing the aperture I got to a limit of 110mm (CO ~0.25) because the companion faded out somewhat earlier. This result is somewhat surprising because so far I consider CO 0.25 as rather equivalent with zero CO but the reduced peak intensity seems to have a toll on fainter companions.

Next opportunity was STF174 (1 Ari, HIP 8544) – 2.9“ +6.33/7.21mag. Easy split with 140mm zero CO and a magnification x70. With x140 then limit aperture 50mm. With 140mm and CO 0.2 resolution somewhat crisper. Could reduce aperture down to 60mm with CO then ~0.46 means a toll of 20% in size of aperture. Seems conclusive.
Wilfried


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Re: First experiments with combined iris and CO mask new [Re: WRAK]
      #6231490 - 12/03/13 01:48 PM

i'm in admiration if not in awe at the depth and expertise in this topic. you guys have really drilled down deep on a complex issue.

my attention is drawn first to the consideration given to "theory" (the animated diffraction profiles are especially enjoyable), and second to the comparison between wilfried's predicted and actual observations.

sidgwick points out in his handbook the several ways that the diffraction artifact in theory differs from its visual appearance in fact. in particular, he states (factually, i think) that the first diffraction ring is brighter than theory predicts, and is in fact often as bright as the airy disk itself.

this should be a red flag to any attempt to take "theory" too seriously as an arbiter of what will appear at the eyepiece, or what is or is not possible visually. for example, it seems we cannot take seriously the theoretical description of the first diffraction ring brightness, but should trust its estimates of radius. personally, i see the first diffraction ring contract with magnitude in roughly the same proportion as the airy disk decreases in diameter with magnitude, so there also theory is in my view inaccurate (although that view, in this forum, is consistently disputed).

there are other aspects of the visual image that deserve scrutiny. for example, diffusion spreads out an "imageless" brightness of the star separate from the spreading into the "imaged" diffraction rings, and this reduces the background contrast that makes faint stars harder to see. (i'm sure most of you have noticed that a 6th magnitude companion that is close but outside the diffraction rings of a brighter star can appear much fainter, more like a 8th magnitude star.)

the spreading out of the light of a companion star inside the diffraction rings also seems unexplained, because the primary star diffraction rings can dominate the image in a way that seems out of proportion to the "theoretical" sum of diffraction ring and secondary star brightnesses -- the rings have an almost corrosive effect on any star within their range. (wilfried reports the "surpising" effect of even a small increase in the brightness of the rings.)

the resolution to many of these problems must lie with vision, what it does and how it works, for which we have little reliable information because the color vision literature is generally concerned with different matching or detection problems at very different illuminance and luminance contrast levels with stimuli that are very much more fixed and precisely defined than double stars observed through miles of turbulent and diffusing air and defined with parameters of uncertain reliability. (thus, the fact that a close companion appears fainter could be because the bright primary reduces the foveal darkness adaptation. i don't know of any vision research that can answer that uncertainty, and exclude diffusion as an alternative explanation.)

add to that the fact that the observer's visual system is itself adapting to repeated and consistent stimulus presentations. in my experience, i found i could readily split pairs after a year of double star observing that i had at first found impossible. partly this was because my skill in using my equipment had improved, partly because i had learned how to look -- how selectively to ignore and how to wait for the best possible image.

couteau mentions somewhere that it takes about a year for the observer to come fully up to speed with a new telescope. and since he doesn't mention that the telescope itself has changed, he must be describing the observer's skill in using the telescope and in interpreting its image -- the perceptual and cognitive components of visual experience.

wilfried and fred among others seem to be keeping exceptionally disciplined observing notes, and i'd guess that if they reported on stars they have observed repeatedly over at least one year, they will say that both the difficulty and appearance of the star has changed, holding constant things like seeing and instrument and magnification.

the fact that different observers in this topic report different perceptions of the same double star does not strike me as a problem to be solved but as an example of the large individual differences in perception.

in most situations i don't necessarily take "more acute" reports of double star resolution or detection, or double star color, as indicating greater skill or sensitivy. just the opposite. lowell's report of martian canals, and antoniadi's failure to confirm them, illustrates that a failure to see something can actually indicate a greater skill at seeing what is actually there. the "competitive" aspect of "being able" to split or detect a close pair can bias an objective assessment of the image.

skill, motivation, experience, bad data -- these seem to me to be the obstacles to any "rule" attempting to "predict" whether unequal doubles can be visually detected or resolved. i use my personal "rule of thumb" with the emphasis on the thumb, less on the rule. i don't apply the rule to decide whether or not i will see something, but to alert me to something exceptional in the observing problem: worsened seeing, not optimal magnification, defocus, diffusion contrast -- or bad positional and magnitude data. in other words, the rule is context i can use to troubleshoot the observing challenge.

the RoT i use -- at more than one magnitude difference between binary components, the minimum resolvable separation is equal to the approximate resolution limit of the scope times the magnitude difference, or R*deltaM -- is pretty conservative. it says i cannot see many things that i can, and in that way not presume through greater accuracy to place a fixed limit on my ability to improve my observing skills.

i learned a lot from the many detailed points made here ... my thanks.


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Re: First experiments with combined iris and CO mask new [Re: drollere]
      #6232180 - 12/03/13 08:53 PM Attachment (6 downloads)

Quote:

i see the first diffraction ring contract with magnitude in roughly the same proportion as the airy disk decreases in diameter with magnitude, so there also theory is in my view inaccurate (although that view, in this forum, is consistently disputed).



I am always interested in this effect, not so much to dispute it but to understand and debate it. I tend to observe the first ring dim noticeably on dimmer stars and it certainly appears smaller. If we assume the central disc is actually subtending a smaller angular diameter with dimmer magnitude, then the ring would certainly appear to contract in radius, too. The first ring on stars that are just bright enough to show a first ring does appear to have about the same dark space surrounding it as a brighter star.

I am not sure dimmer stars bright enough to show a first ring actually subtend a smaller angular diameter even though they appear smaller. It seems reasonable to observe them shrinking inversely with magnitude, but I think this is really a pretty well documented human physiological phenomenon. So, if dimmer stars are in fact still at 1.22 Lambda/D for the Airy disc, per theory, and closer to Lambda/D for the visible disc, then theory can hold for that magnitude range. Surely, once a star falls off in magnitude sufficiently, the edges of the PSF certainly do fall below the visible threshold and the visible diameter of the star does subtend a smaller angular diameter with the Airy disc remaining constant. No ring is likely visible at this magnitude range.

For example, in accord with Sidgwick, a first magnitude star's first ring certainly does dazzle and is almost as bright as the central disc itself. Falling in magnitude, the first ring seems to fade at a slightly quicker pace than the disc. This is probably the human eye's response to less "dazzle" effect, which does need to be accounted for in any RoT. It's "apparent" brightness can affect, IMO, the visibility of a companion nearby. Say, for bright stars magnitude difference of 3 is tough when the companion is on the first ring. But, this also holds for dimmer primary stars, it seems. So something seems to hold true as apparent brightness falls off.

It's interesting that relatively bright stars appear very dim near a brighter companion. They often appear more of a speck (or a broadening of the ring) than an actual disc. Another interesting thing is observing dim doubles. They appear smaller in angular diameter and in angular separation. However, on closer inspection, it can be seen that both companion and primary are indeed inside the first ring (which might have slightly elongated ring, but very dim to see clearly.) I think STT 96 is a good example, since it's separation is near 0.9" arc, it suggests the ring is still very near where theory would predict it (at about 1.2" arc in my own scope.)

Quote:

this should be a red flag to any attempt to take "theory" too seriously as an arbiter of what will appear at the eyepiece, or what is or is not possible visually.



Yea, there does seem to be something more happening here than pure diffraction theory in determining the apparent diameter of close (equal and unequal) double stars in terms of radii. But, I think that something is human physiological theory and it should not be affecting real angular measurements. And if delta magnitude of three proves to be consistently difficult near the first ring among samples of varying brightness, human perception may not be playing a role either in the actual performance but, instead, out perception of that theoretical performance. IME, both 42 Ori and BU 396 are near delta magnitude ~3 with a companion near the ring. They are approximately the same difficulty, with 42 Ori maybe somewhat more difficult. It is brighter and it's ring dazzles more so that BU 396, but it's companion is also slightly dimmer. So one would expect it to be a little more difficult.

It's an interesting topic and I do not pretend to have it entirely right nor fully understand it. But such discussion is a journey of understanding.


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azure1961p
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Re: First experiments with combined iris and CO mask new [Re: Asbytec]
      #6232510 - 12/04/13 12:27 AM

What could be exceptionally revealing here in the effect of a dimmer companion within or near the diffraction rings would be to test this with an artificial star (or two rather). Suiter mentions for an 8" aperture for my reflector that Id use an Christmas tree ball (silver) at a size of 30mm at a distance of 146 feet from my reflector. The sun reflected from this bauble would subtend a true diffraction pattern (suitable for star testing my 8" f/9).

But what if we adjusted things a bit?

Two Xmas tree balls placed together at a specified distance could create a Dawes split. Add a strip of wratten film over one of the balls to dim it so its now a double that's glimmering dimly within the *primaries* diffraction rings? By being able to MANIPULATE these patterns the while RoT could take on an explorative path not so readily found in the sky. Illusions created could be literally isolated and taken apart - can't do that with a true double star.

Another method Suiter mentions - but it sens more technically difficult to fabricate - are measured pinholes at distances - perhaps much farther than the Xmas balls.

Wouldn't it be a fine thing though to *sim* a double like that? Again - gelatin filters cut to size and placed accordingly to dim the companion then observing the effects . A gentle breeze blowing the *companion* through the rings like a pendulum could be quite enlightening indeed. Then removing the primary altogether to see the companion show forth without interference.

Just some thoughts. I'm enjoying this thread.

Pete


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Re: First experiments with combined iris and CO mask new [Re: azure1961p]
      #6232701 - 12/04/13 04:25 AM

Bruce, thanks for your positive comments and interesting propositions.
Language is tricky and I would like to discuss two of the used terms:
- "Rule" - it seems to me that you and probably most of us are using this term expecting a deterministic result. Same is the case for the terms "limit" and "criterion". When studying the different approaches for a RoT for resolving unequal binaries I found the paper of Napier-Munn very interesting - not so much for his results (his algorithm produces errors for specific parameters) but for his concept of calculating probabilities on base of statistical analysis. This concept made immediately sense for me as there are so many factors involved that it seems impossible to include all parameters with (and this is the next challenge) the "correct" values. Using the concept of probability you can suddenly cover all parameters not directly considered and even data errors and still make useful conclusions. Meanwhile I think this probability approach is also to apply for all other limits and criterions like Dawes, Sparrow and even Rayleigh and further on for telescope magnitude limit and so on to make sense. So in my language the probability concept is applied to all rules, limits and criterions discussed here in the field of double star observing
- "Airy disk" - I have the impression that we have here a confusion between "Airy disk" (= first minimum) and the central or spurious disk (= the part of the Airy disk we can see). The latter changes size with magnitude and it may be a trick of the brain that then also the size of the first ring seems to change. But may be some time I will be ready to make measurements with an astrometric eyepiece.
Wilfried


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Re: First experiments with combined iris and CO mask new [Re: WRAK]
      #6232771 - 12/04/13 06:16 AM

Pete, I'm thinking varying pin holes at long distances, you can do the math and set them precisely. But, that would be more lab like instead of actual double observing. Still...

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