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WRAK
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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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WRAK
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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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WRAK
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Reged: 02/18/12

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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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WRAK
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Reged: 02/18/12

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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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Asbytec
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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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WRAK
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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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Asbytec
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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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azure1961p
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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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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
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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Asbytec
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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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DesertRat
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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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Asbytec
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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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DesertRat
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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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WRAK
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Reged: 02/18/12

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Re: Next experiments with CO mask [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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DesertRat
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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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Asbytec
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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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azure1961p
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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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WRAK
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Reged: 02/18/12

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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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DesertRat
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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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DesertRat
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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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