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Equipment Discussions >> Reflectors

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Astrojensen
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Re: Coma and magnification--a mystery new [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
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Re: Coma and magnification--a mystery new [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
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Re: Coma and magnification--a mystery new [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
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Re: Coma and magnification--a mystery new [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
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Re: Coma and magnification--a mystery new [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
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Re: Coma and magnification--a mystery new [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
- Telrad
- 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
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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
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma and magnification--a mystery new [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
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Re: Coma and magnification--a mystery new [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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Astrojensen
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Re: Coma and magnification--a mystery new [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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nevy
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Re: Coma and magnification--a mystery new [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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FastMike
sage


Reged: 09/22/10

Loc: Florida 27° 15' N
Re: Coma and magnification--a mystery new [Re: Starman1]
      #5665399 - 02/06/13 02:32 PM Attachment (21 downloads)

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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Starman1
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Re: Coma and magnification--a mystery new [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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Starman1
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5665458 - 02/06/13 02:59 PM Attachment (18 downloads)

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

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Starman1
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Re: Coma and magnification--a mystery new [Re: Starman1]
      #5665462 - 02/06/13 03:01 PM Attachment (17 downloads)

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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Starman1
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Re: Coma and magnification--a mystery new [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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FastMike
sage


Reged: 09/22/10

Loc: Florida 27° 15' N
Re: Coma and magnification--a mystery new [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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star drop
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Re: Coma and magnification--a mystery new [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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Astrojensen
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Re: Coma and magnification--a mystery new [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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jpcannavo
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Re: Coma and magnification--a mystery new [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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