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Coma and magnification--a mystery

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#26 Astrojensen

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Posted 05 February 2013 - 03:52 PM

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! :grin: )

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
- Telrad
- New secondary holder

And on the to-do list

- A new secondary mirror (the old one has bad astigmatism :vomit: )
- 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

#27 csrlice12

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Posted 05 February 2013 - 03:52 PM

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

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

#28 Jon Isaacs

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Posted 05 February 2013 - 03:56 PM

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

#29 Astrojensen

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Posted 05 February 2013 - 03:59 PM

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

#30 Astrojensen

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Posted 05 February 2013 - 04:09 PM

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... :grin:


Clear skies!
Thomas, Denmark

#31 nevy

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Posted 05 February 2013 - 04:23 PM

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. :lol:

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.

#32 FastMike

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Posted 06 February 2013 - 02:32 PM

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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#33 Starman1

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Posted 06 February 2013 - 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

#34 Starman1

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Posted 06 February 2013 - 02:59 PM

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

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#35 Starman1

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Posted 06 February 2013 - 03:01 PM

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:

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#36 Starman1

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Posted 06 February 2013 - 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.

#37 FastMike

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Posted 06 February 2013 - 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.

#38 star drop

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Posted 06 February 2013 - 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.

#39 Astrojensen

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Posted 06 February 2013 - 06:10 PM

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


Sounds *very* familiar... :tonofbricks:


Clear skies!
Thomas, Denmark

#40 jpcannavo

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Posted 07 February 2013 - 06:28 AM

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.

#41 Eddgie

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Posted 07 February 2013 - 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.

#42 Jon Isaacs

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Posted 07 February 2013 - 08:34 AM

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

#43 Starman1

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Posted 07 February 2013 - 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.

#44 GlennLeDrew

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Posted 07 February 2013 - 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.

#45 jpcannavo

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Posted 07 February 2013 - 01:20 PM

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.

#46 Jon Isaacs

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Posted 07 February 2013 - 01:56 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.



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

#47 GlennLeDrew

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Posted 07 February 2013 - 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...

#48 Jim Romanski

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Posted 08 February 2013 - 06:19 PM

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.

#49 Mark Harry

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Posted 09 February 2013 - 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.

#50 Jarad

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Posted 09 February 2013 - 08:33 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.


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

Jarad






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