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Coma and magnification--a mystery

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#51 CHASLX200

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Posted 09 February 2013 - 12:11 PM

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

Chas

#52 JCB

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Posted 09 February 2013 - 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.berkel..._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

#53 JCB

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Posted 09 February 2013 - 08:50 PM

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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#54 Mark Harry

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Posted 10 February 2013 - 07:04 AM

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

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Posted 10 February 2013 - 04:50 PM

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

#56 Mark Harry

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Posted 10 February 2013 - 05:28 PM

Yeah, sorry, I missed that. Appologies.
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#57 Starman1

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Posted 10 February 2013 - 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.

#58 jpcannavo

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Posted 10 February 2013 - 09:39 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.berkel..._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?



#59 JCB

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Posted 11 February 2013 - 04:37 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.


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.


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

#60 wh48gs

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Posted 11 February 2013 - 04:39 PM

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

#61 JCB

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Posted 11 February 2013 - 04:43 PM

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.


Your explanation sounds very logical. However, I've noticed strange things when testing binoculars. The situation is similar: large pupils (4 – 5 mm), with aberrations in the instrument, coma being very common (easy to check by the use of a small monocular behind the eyepiece to magnify the image). I expected that small defects of binoculars would be easily detected with my left eye, because it is slightly better than my right eye. In fact it's the opposite, as if my left eye were more tolerant for small aberrations. My only explanation is that the optical aberrations of my right eye and the aberrations of the instrument are combined to damage the image.

Visual acuity at large pupils is probably a complex function of eye aberrations, density of cones, and properties of the retina.

I should try the same experiment with my telescope, and verify if coma appears differently in my two eyes at low powers.

Jean-Charles

#62 Starman1

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Posted 11 February 2013 - 04:48 PM

I should try the same experiment with my telescope, and verify if coma appears differently in my two eyes at low powers.

Jean-Charles

Good call. I've never done this. It should be an interesting experiment.






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