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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Coma and magnification--a mystery
#5662033 - 02/04/13 03:54 PM

So here is the optical explanation for the visibility of coma in a short f/ratio newtonian reflector as revealed by the eyepiece:

Coma grows larger in a linear fashion from the center. A very short distance out from the center, the amount of coma still lies within the Airy disc and is not visible. Outside of that, coma steadily increases until the size of the comatic star image crosses a visible threashold.

Ergo, if we take 4 20mm eyepieces, of 50 degree, 68 degree, 82 degree, and 100 degree, the wider the apparent field, the more visible coma should be. The wider apparent field extends further out into the focal plane of the scope, and reveals star images of more coma. If I multiply correctly, the size of a comatic star image at the edge of a 100 degree eyepiece should be twice as long, radially, as a comatic star image at the edge of a 50 degree eyepiece.

Let's hypothetically double the magnification, so the true fields seen (i.e. the width of the image seen on the focal plane of the scope) are 1/2 as wide.
The comatic stars at the edge of the field in every eyepiece would be 1/2 as wide.

Yet, the magnification has doubled, making the comatic star images twice as big. When you double the magnification on an object 1/2 as wide, the result is 1, so the comatic star images at double the power should appear identical, as long as the apparent field doesn't change, at all powers. The linear size of the star image decreases, but the apparent size does not.

So if you use a coma corrector at low power, so long as all your eyepieces have the same apparent field of view, you should need the coma corrector all the way to the highest power your scope can produce.

Therein lies the mystery. Many, if not most, people report that the visibility of coma goes down as they raise the magnifications. That doesn't make sense from the standpoint of optics. So is there a reason or group of reasons why that might pertain?

I've thought about it and came up with this:
1) since the comatic star image behaves a little like an extended object, as the magnification is raised it grows fainter. Since the outer parts of the star image in a comatic star are fainter than the inner parts of the star image, perhaps they fade to invisibility. This might especially be true if the scope is being used in a light-polluted environment.
2) Many people have narrower apparent field eyepieces for high power than they do for low power. The objects being viewed are typically smaller, and many people do not feel pressed to pay the big bucks to maintain the ultrawide fields as the powers go up and the objects they're viewing get smaller. Narrower apparent fields, remember, show smaller comatic star images at the edge.
3) People look at the edge in a low power eyepiece because they're looking at big objects--star clusters, nebulae--and so they see the coma at low power. At high power, they're looking at smaller objects and not paying attention to the edge at all.
4) The eye requires a larger apparent size to see faint details as the object gets fainter. As the apparent size of the comatic star image stays the same at all powers (given equal apparent fields in the eyepieces), perhaps the fall off in apparent brightness of the outer edges of the comatic star image actually requires MORE magnification to make it visible (or, in another sense, an even larger apparent field) as the magnification goes up.
5) perhaps the higher-power eyepieces display less field curvature than the lower power eyepieces because of their smaller field stops. Field curvature could cause stars at the edge of the field to grow in size because they would be ever-so-slightly out of focus compared to the stars in the center. That would make the apparent size of the stars at the edge smaller at higher powers.

Perhaps we've all stumbled onto a psycho-physiological phenomenon that is related to the way a brain sees an image. I'm not sure of the answer as to why coma is found to be less visible at higher powers. Even some older texts say the same thing.

I did a recent test of my 12.5" f/5 scope without a coma corrector, and since it's been so many years since I viewed without coma correction, I wasn't surprised to see coma at all magnifications. And, I thought, it was pretty noticeable at all magnifications. I can't say it seemed the same at all magnifications, but i did find it objectionable at all magnifications all the way to a 1mm exit pupil, so count me as one who WILL use a coma corrector for all eyepieces.

Does anyone else have a different reason for why many observers don't see as much coma at higher powers? Or, perhaps, the EXACT reason why so many observers don't? I don't exactly feel like Sherlock Holmes, but it is a mystery.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Coma and magnification--a mystery [Re: Starman1]
#5662185 - 02/04/13 05:37 PM

A very cogent analysis, Don. I'm sure all your listed contributions to the seeming reduced visibility of coma at higher magnifications come into play at one time or another. You might add another:

6) As field size decreases, statistically there will be fewer bright stars contained within.

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Tim L
Carpal Tunnel

Reged: 12/17/08

Loc: Missouri
Re: Coma and magnification--a mystery [Re: Starman1]
#5662195 - 02/04/13 05:44 PM

Don,

Thanks for posting this. I picked up a GSO coma corrector a while ago, but haven't yet got much use out of it. I tried testing it with a low-power EP (as per the conventional wisdom), but my eye's astigmatism is so bad at that exit pupil, I couldn't recognize any improvement.

After reading this and other posts, I'm going to try again with a 10mm 82-deg EP and slightly defocus to see if I can see the oval coma shape off axis. I'd like to be able to figure out the spacing for visual use on my coma corrector.

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: Tim L]
#5662212 - 02/04/13 05:55 PM

Quote:

Don,

Thanks for posting this. I picked up a GSO coma corrector a while ago, but haven't yet got much use out of it. I tried testing it with a low-power EP (as per the conventional wisdom), but my eye's astigmatism is so bad at that exit pupil, I couldn't recognize any improvement.

After reading this and other posts, I'm going to try again with a 10mm 82-deg EP and slightly defocus to see if I can see the oval coma shape off axis. I'd like to be able to figure out the spacing for visual use on my coma corrector.

One of the primary reasons, I believe, why some people find coma unnoticeable or unobjectionable in low power eyepieces in scopes like f/4.3 or f/4.5 is what you point out--astigmatism so dominates the stars at the edge that coma is insignificant as an aberration.

If you use the ovality test at the edge of the field, make sure it's a relatively bright star you test, and ignore the fact the secondary shadow will appear off-center in the star image. By the way, in many or most eyepieces the star will not appear completely round when defocused at the edge because the eyepieces themselves are not perfectly corrected in either astigmatism or distortion. When you do this, you'll probably settle for the "least oval" solution, You may not be able to find a completely round edge-of-field star image at any setting of the eyepiece-coma corrector distance. That won't matter much when in focus.

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Vic Menard
Post Laureate

Reged: 07/21/04

Re: Coma and magnification--a mystery [Re: Starman1]
#5662213 - 02/04/13 05:56 PM

Well, I use a coma corrector in my f/4 Dobsonian at all magnifications. Coma and field curvature are first on my hit list, but the underlying reason is the synergistic gain when paired with eyepieces that are designed for use with the coma corrector.

Perhaps at higher magnifications the brighter background stars are less numerous. I've read a few articles describing the best "richest field" telescope and, given a fixed AFOV, depending on the threshold magnitude of the dimmest background stars (easily seen, seen with scrutiny, or seen with averted vision...) the size difference varies considerably. In this respect, the observer preference defines the optimal aperture. I suspect coma detection could fit in a similar category.

And then there are observers who feel coma correction isn't necessary until the focal ratio falls below f/4.5. I think those same observers might call me "picky".

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johnnyha
Postmaster

Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Coma and magnification--a mystery [Re: Vic Menard]
#5662519 - 02/04/13 09:27 PM

Could it have anything to do with the diminishing size of the field stop?

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FirstSight
Postmaster

Reged: 12/26/05

Loc: Raleigh, NC
Re: Coma and magnification--a mystery [Re: Vic Menard]
#5662553 - 02/04/13 09:50 PM

Quote:

Well, I use a coma corrector in my f/4 Dobsonian at all magnifications. Coma and field curvature are first on my hit list, but the underlying reason is the synergistic gain when paired with eyepieces that are designed for use with the coma corrector.

Well, Vic if my assumption is correct that you're at Winter Star Party at this very moment, perhaps you'll have some opportunity to field-test some of Don's theories (or your own) about the relationship of coma vs focal-ratio. Surely a half-hour sometime experimenting without a coma corrector in your scope might be a worthwhile sacrifice in the interest of science. I'd try some experimenting myself, but unfortunately I'm stuck at home in N.C. this year instead of WSP, and we're socked in solid under clouds tonight. But next suitable clear night out, I plan to experimentally dabble a bit with this issue, though I don't have quite the technical sophistication of you or Don to grasp the nuances of what I might be seeing...

Edited by FirstSight (02/04/13 09:52 PM)

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: johnnyha]
#5662811 - 02/05/13 01:23 AM

Quote:

Could it have anything to do with the diminishing size of the field stop?

Yes, the field stop diminishes in size as you raise magnification, keeping apparent fields the same.
But the increase in magnification merely expands the apparent size of the comatic star back up to the same size to the eye as the more comatic star at lower magnifications.
If a star at the edge of a 50 degree field at 100X is 0.2mm wide to the eye, the star at the edge of the field at 200X is 0.1mm wide (half the linear width) times 2 (double the magnification) = 0.2mm wide to the eye.
Yet people say coma is less objectionable at high power. That's the mystery.

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Astrojensen
Post Laureate

Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Coma and magnification--a mystery [Re: Starman1]
#5662888 - 02/05/13 03:48 AM

When the true field decreases in size with the increase in magnification, at one point you'll hit a size where all stars across the field are resolved airy disks and coma is small enough to be hidden inside them. The maximum true field diameter at which point this happens depend on the f/ratio and the magnification where it happens depends on the diameter of the field stop of the eyepiece and the apparent field of said eyepiece and the according focal length.

Clear skies!
Thomas, Denmark

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Nils Olof Carlin
Pooh-Bah

Reged: 07/26/04

Re: Coma and magnification--a mystery [Re: Starman1]

hi Don,

One possible explanation of the paradox: it may be a matter of transition from the pure domain of wave optics in the center of the field where aberrations are small in terms of RMS wave aberrations, to the edges of a low-power field where the wavefront deviation is many wavelengths and the image is well represented by ray optics, and the comatic image will form the classic "ice-cream cone" image of coma in ray-traced images.
In your example of f/5, coma wavefront error is within 1/14 wave RMS in a circle of 2.8 mm diameter - here the airy disk will have more or less accentuated and skewed rings, brighter to one side, but not an elongated "coma" image, as farther out.

This image gives some idea of what to expect - the images, for f/5, are at 0, 0.7, 1.4 and 3.5 mm from the center of the field. Only the last image begins to look like coma. Assuming a 1 mm exit pupil, this is for 0, 8, 16, 40 deg off center. Only in the last image is coma obvious as such.

Within the 2.8 mm dia circle, the wavefront error will of course affect planetary contrast just as a similar wavefront error from any other mirror aberration, but star images would not immediately suggest coma unless you look for the asymmetric ring.

You write: " the amount of coma still lies within the Airy disc and is not visible." This is a classic mixing of metaphors - the Airy disk is purely wave optics, the rays that are projected from the mirror to within or outside the disk is something entirely else. This is not uncommon shorthand, and things scale nicely, but it should not be taken literally. For "diffraction limited" optics, in any reasonable sense, wave optics are mandatory, as is wavefront error.

Nils Olof

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Sasa
professor emeritus

Reged: 11/03/10

Loc: Ricany, Czech Republic
Re: Coma and magnification--a mystery [Re: Starman1]
#5662918 - 02/05/13 04:50 AM

Don, I was wondering about it some time ago as well. I think, there are two main contributions. As was already mentioned, there are much less bright stars in the field when using higher magnifications.

The other reason is seeing. You compare coma to airy disc size, but with large dobsonians (I mean 200mm and more) your resolution is more limited by the atmosphere. At high magnification your scale is the blured star while at low magnifications the effects of atmosphere are negligible. So the visual "coma-free" regions is not anymore linear function of magnification.

BTW, at those times I derived a coma equivalent of the "2D[mm] rule" for maximum useful magnification. The coma in Newton should become visible if you are f*f/2 degrees out of axis. So for example, coma in my former f/5 Newton should become visible when you are 13 degree of the axis, and in my former f/6.4 Newton when you are 20 degrees away (Nagler).

My favourite object of checking coma was Trapez. For example in N150/750, coma was clearly visible from about half of the FOV of Panoptic 19mm.

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma and magnification--a mystery [Re: Nils Olof Carlin]
#5662943 - 02/05/13 05:46 AM

Quote:

Within the 2.8 mm dia circle, the wavefront error will of course affect planetary contrast just as a similar wavefront error from any other mirror aberration, but star images would not immediately suggest coma unless you look for the asymmetric ring.

This is something I have noticed at the eyepiece. At higher magnifications, the effect of coma is visible as reduced off-axis planetary sharpness/contrast/detail, i.e. a smaller sweet spot.

I also agree with the Glenn's comment about the reduced probability of there being bright stars at the edge. When I want to observe aberrations like coma, astigmatism and/or field curvature, I start with a reasonably bright star and then move it off-axis to inspect it.

Jon

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: Astrojensen]
#5663278 - 02/05/13 10:23 AM

Quote:

When the true field decreases in size with the increase in magnification, at one point you'll hit a size where all stars across the field are resolved airy disks and coma is small enough to be hidden inside them. The maximum true field diameter at which point this happens depend on the f/ratio and the magnification where it happens depends on the diameter of the field stop of the eyepiece and the apparent field of said eyepiece and the according focal length.

Clear skies!
Thomas, Denmark

Yes, but the zone where coma is contained within the Airy disc is only 2.22-2.75mm at f/5 and smaller for eavery f/ratio shorter. Since even minimal fields of view in eyepieces are substantially wider, this is not the issue.
Of course, 43 degree field Orthos do have very small field stops in shorter focal lengths.
But people who use only orthos in a dob may never notice coma, either.

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: Nils Olof Carlin]
#5663321 - 02/05/13 10:42 AM

Nils Olof,

Thanks for correcting how I stated the initial problem. I should have said that coma is unlikely to be visible at all when the smear of the star image doesn't result in any eccentricity of the diffraction pattern, but you understood what I meant.

The issue, here, isn't where in the field coma becomes visible, but why higher power eyepieces of identical apparent field don't display coma to most people when lower power eyepieces do.

I like the dearth of stars in the field idea, because it is so obvious.

But I'm beginning to think that doubling the size of a comatic star image 10mm off axis does not result in exactly the same appearance of that star that the comatic star image 20mm off axis has at half the power. Your illustration seems to show that and I think I need to spend a little time with some math of the comatic images to see if the apparent width of the star image is identical in both cases. It should be.

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: Starman1]
#5663334 - 02/05/13 10:48 AM

As an aside, Nils Olof Carlin's illustration shows well why coma is a more commanding problem the wider the apparent field of the eyepiece gets.
And the difference in eyepieces used was one of the reasons I thought many people don't see coma at higher powers.

Enter TeleVue, where the higher-power Ethos actually have WIDER apparent fields than the lower power Ethos. Not a good prescription for reducing the presence of coma at higher powers.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Coma and magnification--a mystery [Re: Starman1]
#5663371 - 02/05/13 11:05 AM

Don,
Did you mean higher power Ethos vs lower power Nagler (or other design)?

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Darren Drake
Carpal Tunnel

Reged: 10/09/02

Loc: Chicagoland
Re: Coma and magnification--a mystery [Re: GlennLeDrew]
#5663410 - 02/05/13 11:28 AM

Glenn,
I think Don may have been refering to the 3.7mm and 4.7mm Ethoi since they have 110 degree fov instead of the 100 degree fov.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Coma and magnification--a mystery [Re: Darren Drake]
#5663427 - 02/05/13 11:36 AM

Darren,
Of course! He did specify *apparent* FOV, while I'm sure I must have been stuck on true FoV.

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma and magnification--a mystery [Re: Starman1]
#5663493 - 02/05/13 12:16 PM

Quote:

Yes, but the zone where coma is contained within the Airy disc is only 2.22-2.75mm at f/5 and smaller for eavery f/ratio shorter. Since even minimal fields of view in eyepieces are substantially wider, this is not the issue.
Of course, 43 degree field Orthos do have very small field stops in shorter focal lengths.
But people who use only orthos in a dob may never notice coma, either.

I think Nils's point was that even within that "coma free"/ diffraction limited circle, the coma does affect planetary views. I believe the coma free region is defined as the region where the Strehl has dropped from 1.00 to 0.80..

I think of my fast Newtonians as Catadioptic telescopes..

Jon

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dscarpa
Carpal Tunnel

Reged: 03/15/08

Loc: San Diego Ca.
Re: Coma and magnification--a mystery [Re: Starman1]
#5663603 - 02/05/13 01:37 PM

Thanks for all the info! I talked to Rob last night and will be getting SIPS with my 11" F/5 STS. It should be a plus for lunar viewing too. David

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Astrojensen
Post Laureate

Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Coma and magnification--a mystery [Re: Jon Isaacs]
#5663696 - 02/05/13 02:42 PM

Quote:

I think of my fast Newtonians as Catadioptic telescopes.

And I think I should get a coma corrector and do the same. Niels' post has been a bit of an eye opener for me, as it showed just how much coma affects resolution and contrast, even in an f/5 newton, where it is normally considered not too big of an issue.

It is pretty dang hard to keep planets in the sweet spot at +180x all the time, unless you have a tracking scope. Hmm. Equatorial platform or coma corrector? I do have a coma corrector for my binoviewer, but not for my single eyepieces.

Clear skies!
Thomas, Denmark

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Astrojensen
Post Laureate

Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Coma and magnification--a mystery [Re: Sasa]
#5663704 - 02/05/13 02:47 PM

Quote:

The other reason is seeing. You compare coma to airy disc size, but with large dobsonians (I mean 200mm and more) your resolution is more limited by the atmosphere. At high magnification your scale is the blured star while at low magnifications the effects of atmosphere are negligible. So the visual "coma-free" regions is not anymore linear function of magnification.

I think this is true. Seeing will almost always limit the resolution at high magnification.

Clear skies!
Thomas, Denmark

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma and magnification--a mystery [Re: Astrojensen]
#5663760 - 02/05/13 03:20 PM

Quote:

I think this is true. Seeing will almost always limit the resolution at high magnification.

It is probably true that most often it does. But probably more often, thermal equilibrium is the problem...

But when the seeing is excellent, the scope cooled down and rock solid thermally....

In any event, Standard Operating Procedure for me is to put a Paracorr in the focuser of any scope faster than F/6. I don't ask myself whether I need it at 400x, I ask myself what might I gain if I were to remove it. About the only thing that makes sense if I want a wider field of view with my lowest power eyepiece.

Jon

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: Jon Isaacs]
#5663765 - 02/05/13 03:21 PM

Quote:

Quote:

Yes, but the zone where coma is contained within the Airy disc is only 2.22-2.75mm at f/5 and smaller for every f/ratio shorter. Since even minimal fields of view in eyepieces are substantially wider, this is not the issue.
Of course, 43 degree field Orthos do have very small field stops in shorter focal lengths.
But people who use only orthos in a dob may never notice coma, either.

I think Nils's point was that even within that "coma free"/ diffraction limited circle, the coma does affect planetary views. I believe the coma free region is defined as the region where the Strehl has dropped from 1.00 to 0.80..

I think of my fast Newtonians as Catadioptic telescopes..

Jon

That zone is, IIRC, 0.0007 inches x the f/ratio cubed (x 25.4 to convert to millimeters).
or, 0.01778mm x F/R^3.
For f/6, the zone is 3.84mm wide
For f/5, the zone is 2.22mm wide
For f/4.5, the zone is 1.62mm wide
For f/4, the zone is 1.14mm wide
For f/3, the zone is 0.48mm wide

In my 12.5" f/5 dob, the image scale on the focal plane is 2.166'/mm
The image of Jupiter will always be smaller than 0.5mm, well within the coma-free zone. But only if held in the center of the field.
In my 31 Nagler, with a 42mm field stop, yielding a 91' field, only the center 4.8' of that field is not compromised by coma. That's less than 0.3% of the total field of view!!!

I, too, regard my newtonian as a catadioptric scope. My highest-power eyepiece has a field stop 10.4mm wide. With a Paracorr, the entire field is essentially coma-free. Without it, only the center 3.8% of the field is free from coma. Since f/5 is about as slow a dob as I'm likely to own, a Paracorr is an essential, IMO.

So let's look at the field stop of a distortion-free orthoscopic in 5mm (1mm exit pupil). That field stop in a distortion-free 43 degree field is 3.94mm wide.
A 2.22mm coma-free zone dominates the AFOV, and coma is not likely to be bad anywhere in the field, even though it is there.

So, once again, it seems that AFOV helps determine the impact of coma.
You and I like wide eyepieces.....and coma correctors.

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: Astrojensen]
#5663791 - 02/05/13 03:37 PM

Quote:

Quote:

I think of my fast Newtonians as Catadioptic telescopes.

And I think I should get a coma corrector and do the same. Niels' post has been a bit of an eye opener for me, as it showed just how much coma affects resolution and contrast, even in an f/5 newton, where it is normally considered not too big of an issue.

It is pretty dang hard to keep planets in the sweet spot at +180x all the time, unless you have a tracking scope. Hmm. Equatorial platform or coma corrector? I do have a coma corrector for my binoviewer, but not for my single eyepieces.

Clear skies!
Thomas, Denmark

Thomas,
Though it's a segue from the original post, I think you would find a coma corrector like the Paracorr beneficial in several ways:
--expanding the coma-free zone
--making fainter stars more visible everywhere in the field of view
--improving star images by reduction in coma and field flattening
--allowing the drift of an object across the field to not seriously damage the image quality
--improving the resolution of star clusters--especially globulars
--allowing the use of even wider field eyepieces
--providing an accessory to which filters attach so you can change eyepieces without changing filters from eyepiece to eyepiece
--improving the image quality from nearly every eyepiece
--revealing whether astigmatism or coma dominates the outer field in an eyepiece
--improving the visibility of small details within a nebula by concentration of the point sources that make up the detail. [I did a test on M27's outer "ropy" tendrils that arch around the perimeter of the fainter sections, and the coma corrector made them more clearly visible and more tightly focused.]
--you can get large refractor images without paying large refractor prices.
--slightly reducing the impact of seeing on star images.

It might be difficult to justify a \$500 accessory for a \$500 scope, but I guess what you'd have afterwards would be a truly magnificent \$1000 scope. Ever price a 10" refractor and mount? Takahashi has one for \$277,000.

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Astrojensen
Post Laureate

Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Coma and magnification--a mystery [Re: Starman1]
#5663816 - 02/05/13 03:52 PM

Quote:

It might be difficult to justify a \$500 accessory for a \$500 scope,

Oh, far from it, I am afraid! I already have way more invested in accesories (not to mention eyepieces) for the Lightbridge than what the scope cost me (OK, it was used, but you get the point! )

Lemme see now:

- Coma correcting barlow corrector thingy for the Baader Maxbright bino (owned the bino for a while, so it doesn't count)
- New focuser
- New secondary holder

And on the to-do list

- A new secondary mirror (the old one has bad astigmatism )
- Extension tube for the ES eyepieces

All this and the ES eyepieces is way more than twice what I paid for the Lightbridge... But it has been worth it! It has a very smooth mirror, only a bit of overcorrection, so the images are extremely high contrast. I am just limited in resolution by the piece of c... secondary.

I want to build a new structure around the primary, it deserves it, but I think I'll wait until I have used the Lightbridge a bit more and found out what I need. I'll need to read Berry and Kriege's book first as well.

Clear skies!
Thomas, Denmark

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csrlice12
Postmaster

Reged: 05/22/12

Loc: Denver, CO
Re: Coma and magnification--a mystery [Re: dscarpa]
#5663817 - 02/05/13 03:52 PM

Not really optically experienced and all this high end math and all is putting me into a coma......

+1 for Paracorrs! Coma Haters of the World Unite!

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma and magnification--a mystery [Re: Starman1]
#5663825 - 02/05/13 03:56 PM

Quote:

So let's look at the field stop of a distortion-free orthoscopic in 5mm (1mm exit pupil). That field stop in a distortion-free 43 degree field is 3.94mm wide.
A 2.22mm coma-free zone dominates the AFOV, and coma is not likely to be bad anywhere in the field, even though it is there.

In an F/5 telescope, I am quite sure the off-axis aberrations in an ortho will be considerably worse than the coma.

Jon

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Astrojensen
Post Laureate

Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Coma and magnification--a mystery [Re: Jon Isaacs]
#5663838 - 02/05/13 03:59 PM

Quote:

But probably more often, thermal equilibrium is the problem...

Yup! It didn't take long before I learned to keep the dob in the shed!

Clear skies!
Thomas, Denmark

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Re: Coma and magnification--a mystery [Re: Jon Isaacs]
#5663858 - 02/05/13 04:09 PM

Quote:

In an F/5 telescope, I am quite sure the off-axis aberrations in an ortho will be considerably worse than the coma.

Actually, my UO orthos work surprisingly well in my 12" f/5. I haven't done an in-depth test of the whole series from 25mm all the way down to 4mm, but I've tried a 5mm on a few occasions and it was not at all bad. Can't remember whether it was sharp all the way to the edge, but it couldn't have been extremely bad, or I would have noticed it.

My 25mm Zeiss microscope eyepieces work extremely well for being four-element König designs. They are much sharper over their 50° fields than the 20mm GSO Superviews are over the inner 50°. The 25mm Zeiss was the very first eyepiece I tried that showed me pure coma near the edge, and not mixed with field curvature and astigmatism. This showed me that pure coma is much smaller than what I thought it was and that it was something that one could live with, if the other edge aberrations were minimized.

At least in a 50° apparent field...

Clear skies!
Thomas, Denmark

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Re: Coma and magnification--a mystery [Re: Starman1]
#5663884 - 02/05/13 04:23 PM

Quote:

Quote:

Quote:

I think of my fast Newtonians as Catadioptic telescopes.

And I think I should get a coma corrector and do the same. Niels' post has been a bit of an eye opener for me, as it showed just how much coma affects resolution and contrast, even in an f/5 newton, where it is normally considered not too big of an issue.

It is pretty dang hard to keep planets in the sweet spot at +180x all the time, unless you have a tracking scope. Hmm. Equatorial platform or coma corrector? I do have a coma corrector for my binoviewer, but not for my single eyepieces.

Clear skies!
Thomas, Denmark

Thomas,
Though it's a segue from the original post, I think you would find a coma corrector like the Paracorr beneficial in several ways:
--expanding the coma-free zone
--making fainter stars more visible everywhere in the field of view
--improving star images by reduction in coma and field flattening
--allowing the drift of an object across the field to not seriously damage the image quality
--improving the resolution of star clusters--especially globulars
--allowing the use of even wider field eyepieces
--providing an accessory to which filters attach so you can change eyepieces without changing filters from eyepiece to eyepiece
--improving the image quality from nearly every eyepiece
--revealing whether astigmatism or coma dominates the outer in an eyepiece
--improving the visibility of small details within a nebula by concentration of the point sources that make up the detail. [I did a test on M27's outer "ropy" tendrils that arch around the perimeter of the fainter sections, and the coma corrector made them more clearly visible and more tightly focused.]
--you can get large refractor images without paying large refractor prices.
--slightly reducing the impact of seeing on star images.

It might be difficult to justify a \$500 accessory for a \$500 scope, but I guess what you'd have afterwards would be a truly magnificent \$1000 scope. Ever price a 10" refractor and mount? Takahashi has one for \$277,000.

I noticed this with the paracor when looking the trapezium at orin neb , in my 12" I can sometimes see the E star but with the parracor in its allways there , if I remember correctly its a red star.

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Re: Coma and magnification--a mystery [Re: Starman1]

Quote:

I can't say it seemed the same at all magnifications

Here is a picture of what coma looks like in my 28" f/2.75 without a paracorr. I used Photoshop and aberrator to make it as close as I could to what was actually seen. This was with Ethos eyepieces and the star Betelgeuse.

As magnification went up the coma changed shape and seemed to merge into the Airy disk. Maybe that's why coma seems less at higher magnifications.

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Re: Coma and magnification--a mystery [Re: FastMike]
#5665424 - 02/06/13 02:43 PM

Mike,
--it looks like the seeing experienced isn't perfect, so the on axis star image bloated at high powers. Otherwise, I wouldn't expect to see larger on-axis star images until passing 700X (25X/inch).
--did you account for the difference in magnification on the star images near the field stop (i.e. apply an increase in size based on magnification)?

Don

Edited by Starman1 (02/06/13 02:45 PM)

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Re: Coma and magnification--a mystery [Re: Starman1]

I took an image of a comatic star 40 degrees off axis from a program simulation:

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Re: Coma and magnification--a mystery [Re: Starman1]

and here is an image from 20 degrees off axis at twice the power, or, at exactly the same scale as the 40 degree image:

Edited by Starman1 (02/06/13 03:04 PM)

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Re: Coma and magnification--a mystery [Re: Starman1]
#5665471 - 02/06/13 03:03 PM

I notice that the 20 degree image seems to be more concentrated toward the point, even when expanded by magnification, than the 40 degree image.
I'm going to have to experiment, but it looks like the 20 degree image, doubled, isn't the same as the 40 degree image.
If confirmed, this might go a long way toward explaining the mystery.

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Re: Coma and magnification--a mystery [Re: Starman1]
#5665498 - 02/06/13 03:22 PM

The seeing wasn't that great, that's why I showed the bloated star. I'm doing this from memory so the actual size is likely off.

In my picture I tried to keep the image scale the same size for the coma star. The bloated star in the picture was probably not really that big in the eyepiece relative to the coma.

I used all the Ethos eyepieces going from 21 to 3.7mm then back down again. It was easy to see the coma change shape as the magnification went up then back down.

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Re: Coma and magnification--a mystery [Re: FastMike]
#5665747 - 02/06/13 05:59 PM

Seeing makes a huge difference in my 25" as far as detecting coma using ES 100° eyepieces. Many nights I can't get pinpoint stars on axis at 181x and on some nights 121x is pushing it. Testing this on globular clusters gets rid of the sparser field at higher magnifications problem.

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Re: Coma and magnification--a mystery [Re: star drop]
#5665768 - 02/06/13 06:10 PM

Quote:

Many nights I can't get pinpoint stars on axis at 181x and on some nights 121x is pushing it.

Sounds *very* familiar...

Clear skies!
Thomas, Denmark

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Re: Coma and magnification--a mystery [Re: Starman1]
#5666515 - 02/07/13 06:28 AM

Quote:

Mike,
--it looks like the seeing experienced isn't perfect, so the on axis star image bloated at high powers. Otherwise, I wouldn't expect to see larger on-axis star images until passing 700X (25X/inch).
--did you account for the difference in magnification on the star images near the field stop (i.e. apply an increase in size based on magnification)?

Don

Hi Don and others,
Been away from things for a while - awesome first child (son) Phoenix!
As for seeing, cant remember the last time it was perfect LOL
Empirical confirmation is fun here but clearly - as Nils pointed out - there is no mystery. From a geometric optic standpoint , coma at a given visual angle off-axis of an otherwise infinitesimal point image is invariant to magnification. BUT we rarely deal with true point sources in the visual field. Given the effects of diffraction (the Airy disk) and much more significantly seeing, star images do magnify thereby increasingly "swallowing" the visual effect of coma with increasing magnification. Moreover, I don't think grossly bloated star images are needed for this effect to be realized. That is, it begins at lower magnifications than we might think, given our visual systems (eye/brain) exquisite sensitivity to softening of detail away from ideal - even when its hard to subjectively quantify the precise degree of said softening. Try this experiment. Observer a star field and note the effects of coma in a wide field EP. Now ever so slightly - but progressively - defocus, and note how significantly the seemingly "coma-free" field seems to expand, as the inflating image increasingly overpowers the comatic effect off axis.

Edited by jpcannavo (02/07/13 07:06 AM)

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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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).

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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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.

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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Re: Coma and magnification--a mystery [Re: JCB]

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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Re: Coma and magnification--a mystery [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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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.

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

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

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Re: Coma and magnification--a mystery [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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Re: Coma and magnification--a mystery [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.

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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Re: Coma and magnification--a mystery [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
Pooh-Bah

Reged: 03/02/07

Re: Coma and magnification--a mystery [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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JCB
sage

Reged: 10/04/04

Loc: France
Re: Coma and magnification--a mystery [Re: jpcannavo]
#5674414 - 02/11/13 04:43 PM

Quote:

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

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Coma and magnification--a mystery [Re: JCB]
#5674428 - 02/11/13 04:48 PM

Quote:

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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