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How good are reflectors for resolving binaries?

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#1 WRAK

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Posted 13 August 2013 - 12:12 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

#2 Erik Bakker

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Posted 13 August 2013 - 01:05 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.

#3 David Knisely

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Posted 13 August 2013 - 02:13 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


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.

#4 PJ Anway

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Posted 13 August 2013 - 05:01 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.

#5 Ed Wiley

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Posted 13 August 2013 - 11:13 PM

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.

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#6 WRAK

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Posted 14 August 2013 - 02:00 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."
...
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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#7 azure1961p

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Posted 14 August 2013 - 06:28 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

#8 fred1871

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Posted 14 August 2013 - 08:59 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.

#9 Ed Wiley

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Posted 14 August 2013 - 10:28 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

#10 WRAK

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Posted 15 August 2013 - 02:58 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

#11 Asbytec

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Posted 15 August 2013 - 06:47 AM

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...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.cloudynig...rd=double&am...

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

"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.

#12 WRAK

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Posted 15 August 2013 - 04:06 PM

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

#13 brianb11213

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Posted 15 August 2013 - 04:46 PM

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.

#14 fred1871

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Posted 15 August 2013 - 10:33 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.

#15 Asbytec

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Posted 15 August 2013 - 11:19 PM

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.)

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#16 WRAK

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Posted 16 August 2013 - 02:47 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

#17 Asbytec

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Posted 16 August 2013 - 05:45 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.

#18 Jon Isaacs

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Posted 16 August 2013 - 07:29 PM

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.

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#19 3c_273

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Posted 18 August 2013 - 01:09 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...ction_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.cloudynig...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.

#20 WRAK

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Posted 01 September 2013 - 11:06 AM

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

#21 azure1961p

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Posted 01 September 2013 - 11:55 AM

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...ction_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.cloudynig...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

#22 WRAK

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Posted 01 September 2013 - 01:59 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

#23 azure1961p

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Posted 01 September 2013 - 03:09 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

#24 fred1871

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Posted 01 September 2013 - 09:04 PM


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?

#25 Asbytec

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Posted 01 September 2013 - 10:50 PM

I suggest you try that and continue exploring the topic.






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