Forums Directory All Topics   Announcements and News General Astronomy Equipment Discussions Astrophotography and Sketching Observing Telescope Specific Forums Speciality Forums Other Swap and Shop CN Feedback and Vendor Announcements International Year of Astronomy 2009 - Forums are now closed. Search Articles and Columns Photos

# Coma and magnification--a mystery Topic Page: 1 2

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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
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.

Home-made and modified binoculars My Gallery (mostly DIY stuff) **UPDATED Jun 22, 2013** Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover
Like (0 likes)
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.

Tim

Z10 dob, flocked

Orion ST-80

"When I consider your heavens, the work of your fingers, the moon and the stars, which you have set in place, what is man that you are mindful of him, the son of man that you care for him?" --Psalm 8:3-4

Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
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".

Like (0 likes)
Could it have anything to do with the diminishing size of the field stop?

Johnny FS152 15" Obsession Classic w/ 14.5" f/4.65 Zambuto

Like (0 likes)
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...

__________________________________________________

Orion XT12i with Swayze-refigured primary & Protostar secondary

Televue NP101 refractor

William Optics Megrez 90 refractor

Universal Astronomics Deluxe Mounts

Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
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

"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori

Like (0 likes)
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

 Photos (1)
Like (0 likes)
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.

Like (0 likes)
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

Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
Don,
Did you mean higher power Ethos vs lower power Nagler (or other design)?

Home-made and modified binoculars My Gallery (mostly DIY stuff) **UPDATED Jun 22, 2013** Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover
Like (0 likes)
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.

Astronomy educator/Sidewalk astronomer

Owner of Astronomy Delight franchise

18 inch f4.42 Dob on eq platform w ST120 f/5 finder

16 LB

12 inch Zhumell Dob

C8 Powerstar

8 inch f/6.9 home made Dob with Seevers optics

William Optics red 10th Anniversary 80mm FD

C8 XLT on Vixen GPDX

PST

ETX 125

26lb eyepiece box

Cernan Space Center astronomer

Member of Northwest Suburban Astronomers

Like (0 likes)
Darren,
Of course! He did specify *apparent* FOV, while I'm sure I must have been stuck on true FoV.

Home-made and modified binoculars My Gallery (mostly DIY stuff) **UPDATED Jun 22, 2013** Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover
Like (0 likes)
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

Like (0 likes)
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

Like (0 likes)
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

"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori

Like (0 likes)
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

"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori

Like (0 likes)
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

Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
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

"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori

Like (0 likes)
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!

My eyepieces are made from the waste product of exploding stars.

10XTi 102XLT ST80A(2" Focuser); President, Eypieces Anonymous, Denver Chapter (Hello, I'm an eyepiece junky, what's your excuse?)

DAS Dark Site

Like (0 likes)
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

Like (0 likes)
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

"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori

Like (0 likes)
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

"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori

Like (0 likes)
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.

Celestron C11+EQ6, skywatcher /Zambuto 12"dob,watchhouse tracking platform, 16"/Zambuto  lightbridge , Ethos's 6,8,10,13,17,21. Naglers 31,26. Lieca ash  zoom, televue 2.5 powermate,2"2x powermate .Paracorr type2. Howie glatter 2" laser & Tublug. Lumicon 2" UHC, 03, H-Beta,
Televue smoothies ( all of em) , Meade 4000 smoothies ( all of em). That'll do for now.

Like (0 likes)
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.

28" Webster/Lockwood f/2.75 - eyepiece height 70"

Florida 27° 15' N

 Photos (1)
Like (0 likes)
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

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
I took an image of a comatic star 40 degrees off axis from a program simulation:

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member

 Photos (1)
Like (0 likes)
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:

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member

 Photos (1)
Like (0 likes)
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.

Don Pensack in Los Angeles
www.EyepiecesEtc.com
12.5" Newt, 5" Mak, 4" Apo.
Sustaining Lifetime IDA member
Like (0 likes)
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.

28" Webster/Lockwood f/2.75 - eyepiece height 70"

Florida 27° 15' N

Like (0 likes)
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.

Ted

Remember to have your clouds spayed or neutered, contrails included.

Like (0 likes)
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

"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori

Like (0 likes)
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.

Like (0 likes)