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Equipment Discussions >> Reflectors

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Eddgie
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5666593 - 02/07/13 07:49 AM

Some of the explination may be related to the energy distribution and the emergence of astigmatism.

The book Telescope Optics has an excellent 3D diagram of the light distribution in a comatic/astigmatic star pattern.

While the comatic/astigmatic tail can expand quite far from the abberated Airy Disk, it is shaped almost like the root flare of a tree. The trunk represents the Airy Disk, and a couple big roots coming out at an angle represent the exetnsion of the comatic tails.

As you first move away from the center of the field, most of the energy is still encircled in a very small area. Until it extends more than a couple of arc minutes of apparent field (the amount required for the dark adapbted eye to actually resolve a shape) we see it is a point.

The further we go out, the more energy is transferred from the Airy Disk into the comatic and astitmatic fans. The Airy Disk grows dimmer but the fan itself doesn't necessarily grow brigher because as the fan developes, the way the energy is distributed further and further into the area away from the Airy Disk that the extensions themselves become dimmer and dimmer.

They are bigger and bigger, but because the energy is spread over a far larger area, the very ends of the extension will simply be below the eye's dark adapted ability to detect.

The dark adapted eye's contast sensitivity threshold is only between 5% and 15% depending on the size of the target and the observer's own contrast sensitivity, which does apparently vary from observer to observer.

Anyway, the comatic blur is always much larger than we can see visually.

But put a camera on it and take a long exposure picture, and you see that the coma is far larger than what we see visually because the amount of energy way out in the fan is too low for our eyes to easily detect.

And this. Most reflectors are only fully illuminated at the center of the field. It is not at all unusual for the designer to let the illumination fall off by 30% in a telescope designed for visual use.

So, once again, as you move further from the optical axis, the energy in the fan is groing and the energy in the Airy Disk is draining away, and the more spread out the fan is, the harder the comatic/astigmatic extensions become to detect at the limits of their extension, but at the same time, they are also growing dimmer because of the off axis illumination of the telescope itself.

I believe some of the reason for the descrepencies lie in these to explinations. The energy distribution is growing larger and larger so the tails are growing dimmer at the tips while they are growing ever larger, and the off axis illumination of the scope itself may be further reduceing the brigtness of that blur at low powers so that we never really see the full extension of the comatic/astigmatic blur.

Again, a long exposure image would show the true extension.

As you use eyepecies with narrower field stops, the comatic/astagmitic blur is smaller, but the light intensity distribution makes it much brighter near the Airy Disk.

An easy experiment to kind of explain this is to drift a star near limiting magnitude towards the field stop of an eyepiece with a very wide true field.

If there is bad coma in the system and the field is not fully illuminated, you will see that the star will literally disappear before it gets even close to the field stop.

Studying some 3D plottings for energy distribution for an abberated star and you quickly realize that spot diagrams doe a poor job of telling you the intensity of the extensions. They can get very long, but that energy gets more and more diffused.

In other words, it is very complex, and much depends on the individual's contrast sensitiviy threshold (how faint against the background do the ends of the tail get before they drop out of visibility), the visual acuity of the individual, and the off axis illumination of the scope used.


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Jon Isaacs
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Re: Coma and magnification--a mystery new [Re: Eddgie]
      #5666636 - 02/07/13 08:34 AM

Quote:

I believe some of the reason for the descrepencies lie in these to explinations. The energy distribution is growing larger and larger so the tails are growing dimmer at the tips while they are growing ever larger, and the off axis illumination of the scope itself may be further reduceing the brigtness of that blur at low powers so that we never really see the full extension of the comatic/astigmatic blur.




Eddgie:

I believe the illumination of the field of view is important. My simple minded way of looking at it that a "poorly" illuminated field of view means that off-axis you are not looking at the full mirror, rather only a smaller central portion which means the effective focal ratio is greater and so one should expect less coma. Another way to think about it is that a poorly illuminated field means that you are masking the aperture as a function of radius...

Since most of the coma comes from the outer portion of the mirror, (a 10 F/4.8 mirror has an 8 inch F/6 mirror hidden inside it with about half the coma), a typical 50% edge illumination level would seem to affect the level of coma present.

Of course at higher magnifications the field is typically fully illuminated and coma would be present at the levels one would expect.

Your comments about the brightness of the coma and the fact that it is difficult to see the tails brings up another issue, aberrations like coma and off-axis astigmatism versus sky brightness. I have often noticed that when the skies are dark and clear, coma and astigmatism are more apparent. Eyepieces that seem reasonably clean in a light polluted backyard are seen to show rather obvious and bothersome aberrations under dark skies. I attribute this to the increased contrast dark skies provide so that coma, astigmatism and field curvature are more easily seen in the same way that a faint galaxy is more easily seen. If you really want to test an eyepiece or scope for off-axis aberrations, dark skies are needed.

Jon


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Starman1
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Re: Coma and magnification--a mystery new [Re: Eddgie]
      #5666946 - 02/07/13 11:49 AM

Ed,
In a number of my other posts in various threads, I've discussed why it is that some observers see significant coma and are bothered by it enough to get a coma corrector at f/5.5 while other individuals don't see it at f/4.5 and are not bothered by it at all.

Obviously, the visibility of the extended comatic star image will depend on:
--aperture
--darkness of sky (bright skies wipe out the outer parts of the comatic star image)
--field curvature in the eyepiece
--the expectations of the observer
--the quality of night vision in the observer
--the visual acuity of the observer (astigmatism, sharpness, etc.)
--the viewing style (i.e. do you look at the edge at all?)
--the types of objects viewed
--the apparent field of the eyepiece
--the f/ratio of the scope
--the brightness of the stars at the edge
--illumination of the edge of field
--the presence of angular magnification distortion that may reduce magnification at the edge of the field
--etc.

Your comments lend themselves to explaining why someone would see coma at all apparent field widths, but wouldn't necessarily see worse coma in a widefield eyepiece.

But my original post concerned why, in eyepieces of equal apparent field, people notice less coma at higher powers. From the standpoint purely of linear diameter, there should be no difference in the visibility of coma at all powers as long as the apparent field is constant.
Indeed, one can make a rational argument for seeing MORE coma at higher powers since the outer edges of the star image will be seen against a darker sky to the eye.
Unless, that is, the star acts like an extended object, in which case magnification might make the outer parts of the star image fainter--to the point of not being seen at all. In that case, the size of the comatic images would appear to shrink at higher powers.

But, I'm also aware that I have a scope that is built to provide not more than 0.3 magnitudes of vignetting at the edge of even my largest field eyepiece and near-zero vignetting at any magnification over 200X. That may not be the case with all telescopes, but I have to believe that most reflectors have very little vignetting of the fields at higher powers, so edge illumination isn't going to be the primary reason why coma that's visible at low powers is invisible at high powers.

There are likely to be many reasons why coma is less visible at higher powers, in the same way there are many reasons why some people see coma and others don't. And perhaps some observers see coma at all powers (like me) and don't think you can dispense with coma correction just because a higher power is used.

I don't REALLY think this is a mystery. There are rational explanations for why so many observers notice coma at low, but not high, powers. I started this thread primarily to see what reasons people could come up with.


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GlennLeDrew
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5667017 - 02/07/13 12:36 PM

Jon,
When illumination falls off with increasing field angle, it is not the case that ever smaller *central* portions of the primary are utilized.

Rather, one side of the primary is clipped, while the opposite side is completely seen, with room to spare. So the aperture takes in something of a "cat's eye" aspect. You can see this by removing the eyepiece, placing your eye near the focal surface, and peering in from near one edge of the focuser (where an image would be formed near the field edge when the field stop is large.)

This effect is more pronounced when the secondary is at or near minimal size, the distance between secondary and focus is minimal, and the field stop diameter is large. The field illumination graph will look less like central plateau with shallow slopes at each side, and more like 'pointy' mountain with steep slopes.


Edggie,
Even if illumination fall-off with field angle is significant, the impact on the visibility of the fainter features is not impacted as badly as one might fear. Both the aberrated star and the sky are dimmed equally, and so contrast is preserved. And the eye's huge dynamic range makes brightness *much* less important than contrast. A diminution of 50% by vignetting, if not excessively abrubt, is rather difficult to discern. A decrease in contrast of 10%, however, is probably easier to detect.

One obtains this contrast increase simply by choosing a brighter star, and one not much more so at that, in order to better study the aberrated pattern.

I really feel that vignetting's contribution to the visibility of aberrations is quite minimal.


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jpcannavo
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5667083 - 02/07/13 01:20 PM

Quote:

But my original post concerned why, in eyepieces of equal apparent field, people notice less coma at higher powers.




And the word "notice" here suggests another, purely psychological, explanation: High power observation tends to (but not exclusively)entail center of field attention - i.e. lunar/planetary/planetary nebula detail etc. Low power observation, instead, tends to entail more widefield attention, clusters, nebular star fields etc. Clearly the "noticing" of coma would be more expected in the latter cases.


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Jon Isaacs
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Re: Coma and magnification--a mystery new [Re: GlennLeDrew]
      #5667141 - 02/07/13 01:56 PM

Quote:


Jon,
When illumination falls off with increasing field angle, it is not the case that ever smaller *central* portions of the primary are utilized.

Rather, one side of the primary is clipped, while the opposite side is completely seen, with room to spare.




I wondered about the exact form the reduced illumination took, my gut feeling was that it was not symmetric. Still, it seems that there would be some reduction in coma due the fact that you are not looking at the entire out portion of the mirror... ???

Jon


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GlennLeDrew
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Re: Coma and magnification--a mystery new [Re: Jon Isaacs]
      #5667460 - 02/07/13 04:47 PM

Jon,
I forgot to affirm that by masking off part of the aperture, and no matter what part, the now smaller area does result in somewhat diminished extent of aberration. An advantage afforded by vignetting, which I'm sure some scope/bino makers take advantage of...


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Jim Romanski
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Re: Coma and magnification--a mystery new [Re: GlennLeDrew]
      #5669544 - 02/08/13 06:19 PM

Quote:

And the eye's huge dynamic range makes brightness *much* less important than contrast. A diminution of 50% by vignetting, if not excessively abrubt, is rather difficult to discern. A decrease in contrast of 10%, however, is probably easier to detect.



Well said Glenn!

This is such an important thing to remember for all visual observations. It's all about contrast.


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Mark Harry
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Re: Coma and magnification--a mystery new [Re: Jim Romanski]
      #5670238 - 02/09/13 07:39 AM

My take, doubling the mag decreases the brightness of any arbitrary star by a factor of 4. Also reducing the flare that's visible, especially the outer area of it.
The dimmer the star or starfield, the less visible the flare will appear as well.
F/4.5 viewing starfields- ok. But without some kind of corrector or accurate mount/drive on planets, forget it.
M.


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Jarad
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Re: Coma and magnification--a mystery new [Re: Mark Harry]
      #5670293 - 02/09/13 08:33 AM

Quote:

My take, doubling the mag decreases the brightness of any arbitrary star by a factor of 4. Also reducing the flare that's visible, especially the outer area of it.
The dimmer the star or starfield, the less visible the flare will appear as well.
F/4.5 viewing starfields- ok. But without some kind of corrector or accurate mount/drive on planets, forget it.
M.




Like Don pointed out earlier, stars are point sources, so increasing mag does not spread their light out and decrease their apparent brightness until you exceed the magnification where you can resolve the airy disk. It does decrease the apparent brightness of the comatic fan, though, since that is big enough to resolve. That's one of the reasons he thinks coma seems less intrusive at higher mag (and I agree with his reasoning).

Jarad


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CHASLX200
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Re: Coma and magnification--a mystery new [Re: dscarpa]
      #5670623 - 02/09/13 12:11 PM

I can't stand the coma in any Newt faster than F/6.

Chas

Edited by CHASLX200 (02/09/13 12:12 PM)


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JCB
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5671402 - 02/09/13 08:43 PM

Don,

Part of the explanation could be the fact that the human eye itself is an optical system, with its own aberrations. When the magnification decreases, the exit pupil diameter increases, and it is well known that optical aberrations of the eye also increase with pupil size.

For example, look at the pictures posted here by Photonovore.

The original document (large !) can be downloaded with this link :
http://vision.berkeley.edu/roordalab/Pubs/Optics_of_the_Eye.pdf


By considering that the receptor is not the eye but the retina, the optical aberrations of the instrument, for example coma, will add to the aberrations of the eye to degrade the quality of the image. So, even if the size of the coma blur due to the telescope is constant, on the retina the resulting image is certainly worse at low magnifications.

Jean-Charles


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JCB
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Re: Coma and magnification--a mystery new [Re: JCB]
      #5671412 - 02/09/13 08:50 PM Attachment (6 downloads)

In order to illustrate my thoughts, I have made some simulations with Aberrator. Their intent is only qualitative.

The first image in the upper raw is supposed to be the point spread function of an arbitrary human eye.
The second image (scope 3) is the coma of the telescope.
The third image is the addition of the two previous wavefronts, and represents the image that forms on the retina. It is obvious that the spikes of the coma pattern are greatly enhanced by the aberrations of the eye.

On the lower raw, the same principle applies, but with the coma pattern rotated by 90°. The effect on the retinal image is the same.


In such a case, it is possible to suggest that the defects of the eye alone, or the coma of the telescope alone, do not degrade the retinal image to a visible extend, but that the addition of both aberrations causes a visible damage to the perceived image.
One way to recover a better image quality is to increase the magnification, in order to reduce the aberrations of the eye. The other way is to use a coma corrector in order to reduce the amount of coma.
Of course, since optical defects of the eye vary greatly from people to people, that would explain why some observers are bothered by coma at low powers more than others.

Jean-Charles


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Mark Harry
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Re: Coma and magnification--a mystery new [Re: Jarad]
      #5671901 - 02/10/13 07:04 AM

Stars are point sources, but comatic flares due to optical abberations are -not-. They subtend a finite area.
M.


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Jarad
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Re: Coma and magnification--a mystery [Re: Mark Harry]
      #5672678 - 02/10/13 04:50 PM

Quote:

Stars are point sources, but comatic flares due to optical abberations are -not-. They subtend a finite area.
M.




Yes, precisely. That was the point I was trying to make.

Jarad


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Mark Harry
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Re: Coma and magnification--a mystery new [Re: Jarad]
      #5672729 - 02/10/13 05:28 PM

Yeah, sorry, I missed that. Appologies.
M.


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Starman1
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Re: Coma and magnification--a mystery new [Re: JCB]
      #5672989 - 02/10/13 08:22 PM

Jean-Charles,
Yes, your point should be added to those I mentioned in my first post.
But it is not necessarily the case, as many people do not have any significant astigmatism that would operate at large exit pupils and not at small ones.
So whereas yours is a valid point and may very well hold sway in a certain percentage of case, I doubt it does in most.

Perhaps, though, there is NOT just one reason but a battery of them that comes into play to explain the lesser visibility of coma at higher powers.

I started the thread to see what explanations might exist, and the thread has been good in that regard. I see coma at all magnifications and was confused by those who don't because I could only figure there had to be some reasons why there was a difference.

All the posts so far have been looking for an explanation why many people DON'T see the same coma at high powers as at low.
I'd like to know what's different about the observers who DO see coma at all magnifications.


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jpcannavo
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Re: Coma and magnification--a mystery new [Re: JCB]
      #5673124 - 02/10/13 09:39 PM

Quote:

Don,

Part of the explanation could be the fact that the human eye itself is an optical system, with its own aberrations. When the magnification decreases, the exit pupil diameter increases, and it is well known that optical aberrations of the eye also increase with pupil size.

For example, look at the pictures posted here by Photonovore.

The original document (large !) can be downloaded with this link :
http://vision.berkeley.edu/roordalab/Pubs/Optics_of_the_Eye.pdf


By considering that the receptor is not the eye but the retina, the optical aberrations of the instrument, for example coma, will add to the aberrations of the eye to degrade the quality of the image. So, even if the size of the coma blur due to the telescope is constant, on the retina the resulting image is certainly worse at low magnifications.

Jean-Charles




But, this effect - if significant - would actually work in reverse. Just as the keener ear more easily discerns poor fidelity in an audio system, noticing off-axis coma is an exercise in visual acuity. Thus with sharper vision, the off axis coma of the primary mirror would stand out more - not less - relative to a sharp on-axis image. Similarly, with decreasing visual acuity, the same amount of off-axis coma would present a less apparent deviation from the consequently softened on axis image.
Excuse the hyperbole, but think of it this way: If you had to visually assess the wide field correction of an optical system (i.e. detect/notice the propagation of aberration with increasing angle off-axis), who would you rather have do it - someone with eyes like Stephen O'Mara, or someone with macular degeneration?



Edited by jpcannavo (02/11/13 08:56 AM)


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JCB
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5674394 - 02/11/13 04:37 PM

Quote:

Jean-Charles,
Yes, your point should be added to those I mentioned in my first post.
But it is not necessarily the case, as many people do not have any significant astigmatism that would operate at large exit pupils and not at small ones.
So whereas yours is a valid point and may very well hold sway in a certain percentage of case, I doubt it does in most.




Just a remark: according to the pdf document I mentioned previously, at large pupils there are much more aberrations than astigmatism. In section 2.11.5, one can read this statement:
"strehl ratios are about 5% for a 5 mm pupil that has been corrected for defocus and astigmatism."
The wave aberration is made of numerous Zernike terms, including higher orders, so the performance of the eye can rapidly decrease with the pupil diameter.
Of course, I don't know the extent of this effect. The other points you listed are valid, and can play a very important role.


Quote:

All the posts so far have been looking for an explanation why many people DON'T see the same coma at high powers as at low.
I'd like to know what's different about the observers who DO see coma at all magnifications.




Perhaps those who don't see coma at high powers don't compare carefully on axis and off axis images, and just evaluate off axis images alone? For example, at high powers, coma can decrease the strehl ratio to, say, 0.7. At the edge of the field, the image is still decent, and since many of us look at the centre of the field, the peripheral degradation is not annoying.
At low powers on the contrary, the small spikes or the small spread of light due to coma is unmistakable around stars, so everyone sees coma.

Jean-Charles


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wh48gs
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5674402 - 02/11/13 04:39 PM

Quote:

All the posts so far have been looking for an explanation why many people DON'T see the same coma at high powers as at low.
I'd like to know what's different about the observers who DO see coma at all magnifications.





There's a central portion of image field produced by a paraboloid where there is no detectable deformation of the central diffraction maxima due to coma. It is about twice the diffraction limited field. Given ep AFOV, this portion of field becomes relatively larger as the ep f.l. diminishes. The effect is that the outer field coma diminishes too in the shorter f.l. ep.

Outside this field, coma deformation becomes visible if it is magnified enough. For shape recognition, average eye requires about 5 arc minutes. But some people will need only 3 or 4, and some others 8 or 9. So the coma tolerance due to this factor can vary up to threefold, possibly more in extreme cases.

We, however, cannot look at the coma alone. Eyepiece astigmatism is the dominant factor in the outer field with the conventional types, and still significant with the corrected widefields. Ep astigmatism, given type, scales with its f.l. which means that its angular size remains nearly constant, regardless of magnification. It is only that longer f.l. ep - with so much wider fields - have more coma added in the outer field, so the combined blur is larger.

As for eye aberrations, they don't add up as an aberration affecting the wavefront converging toward the focal plane. The eye looks at the image formed by the objective, and every point of that image is a point source to the eye. In other words, eye aberrations affect the shape of this image little, but can smear its pattern, if significant.

Vla


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