Author Topic: Coma free zone calculation  (Read 448 times)

writgobetfcoo

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Re: Coma free zone calculation
« Reply #15 on: January 25, 2018, 10:21:03 PM »
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Ah, thinking like an astrophotographer, not a visual observer.

You got me Don

Thanks for the reply around the corrected impact. I did mean "coma is half sized in a linear sense".

All this talk has made me realize how critical a coma corrector's placement has to be to get the best results. I think that is also why I am still struggling with the MN190. The coma correction introduced by the meniscus (or a coma corrector) has a sweet spot for which is was designed. Place it anywhere outside of that "sweet spot" and you will see degraded results, although still better than without it.

Appreciate all the advice

Stanley Edwards

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Re: Coma free zone calculation
« Reply #16 on: January 26, 2018, 01:02:27 AM »
Coenie,at http://www.telescope...cs.net/coma.htm (about halfway down the page), Vladimir S discusses the coma as affected by the aperture stop - not always the edge of the primary, in many cases it is instead the edge of the corrector. For a paraboloid mirror, no difference - for a spherical mirror (k=0), coma is cancelled when the stop is at the radius of curvature. If midway (where the corrector is usually placed in SCT, MN and SN types), coma is halved (compared to a paraboloid of the same dimensions). For a Schmidt camera, the corrector is at the ROC thus cancelling coma fully. Thus, as I understand things, the task of the corrector is not to lower coma but to cancel the spherical aberration of the (spherical) primary. The downside of full coma correction in a Schmidt camera is a twice as tall optical tube (and a larger primary than the aperture-defining corrector).A coma corrector used with paraboloid mirrors is something quite different.

ontoolhaiworl

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Re: Coma free zone calculation
« Reply #17 on: January 31, 2018, 01:34:04 AM »
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A 31mm Nagler would see no coma at all (though you wouldn't use a 31mm in an f/4 scope). For sure, no eyepiece used in the scope would see any coma at all.


?

Why wouldn't you use a 31mm Nagler in an f/4 scope?

Some wouldn't because the exit pupil is 7.75mm..

I do because I measured my dark adapted pupil at about 7.7-7.8mm. I decided to try to measure it because I had noticed that some objects were noticeably brighter in the 41mm Panoptic in an F/5 scope than in the 35mm Panoptic and the 31mm Nagler..

And even if one's pupil is less than the exit pupil, the wide field of view can be useful.

Jon
I don't have an f/4, but if I did I am certain I would be using a Paracorr making it an f/4.6. Exit pupil of the 31 would be 6.7mm, close to perfect for dark sky.

unamprodce

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Re: Coma free zone calculation
« Reply #18 on: February 08, 2018, 12:17:46 AM »
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One might use a Baader MPCC. Or one just might not use a Paracorr. I don't always use a Paracorr at F/4, this thread is, after all, about not using a coma corrector and understanding just how much coma there is to be seen.

Jon

David Felkel

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Re: Coma free zone calculation
« Reply #19 on: February 09, 2018, 05:33:46 AM »
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Coenie,
at http://www.telescope...cs.net/coma.htm (about halfway down the page), Vladimir S discusses the coma as affected by the aperture stop - not always the edge of the primary, in many cases it is instead the edge of the corrector.
For a paraboloid mirror, no difference - for a spherical mirror (k=0), coma is cancelled when the stop is at the radius of curvature. If midway (where the corrector is usually placed in SCT, MN and SN types), coma is halved (compared to a paraboloid of the same dimensions). For a Schmidt camera, the corrector is at the ROC thus cancelling coma fully. Thus, as I understand things, the task of the corrector is not to lower coma but to cancel the spherical aberration of the (spherical) primary.

Nils, refering to your last statement, if the corrector is only correcting spherical aberration, where did the coma go to? I understand spherical aberration to be similar to coma but producing different results. Are we saying that a spherical mirror does not have coma? Since a MN only has the meniscus to correct the image and it manage to produce less coma, is this, from the argument above, meaning to say that the spherical aberration correction being corrected results in less pronounced coma?

The referance article is a bit advanced for my quick reading and understanding so please bear with me. I need to upskill on a lot of the technical terms used first to even begin to make sense of the arguments made.

ryepittimy

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Re: Coma free zone calculation
« Reply #20 on: February 09, 2018, 11:13:57 AM »
Read here:http://www.telescope.../Mak-Newton.htm , specifically, "...coma is reduced to ~30% of that in a comparable paraboloid."