The visibility of coma depends on:
1) the darkness of your sky. The faint outer portion of the comatic star image may not be visible against a bright sky, but will be against a dark sky.
2) the apparent field of the eyepiece. The wider the apparent field, the more visible coma is. In a sense, coma correctors made 100° eyepieces in dobs possible.
3) the presence/absence of astigmatism in the eyepiece. If the eyepiece has lateral astigmatism, it will mask coma, a smaller aberration.
4) the field curvature of the system. I saw horrible coma in a 35mm Panoptic at f/5--really bad--but after the coma was removed, I could see the edge of field stars were slightly defocused.
That made coma appear far worse by defocusing.
5) the presence/absence of astigmatism in your eye. If you have an eyepiece whose astigmatism goes away when wearing glasses, then using glasses will reduce the aberrations seen to
only FC. coma, and lateral chromaticism in an eyepiece that has no inherent astigmatism.
6) your expectations. If, like me, you want star images to be tiny little pinpoints from edge to edge in the field, then you will use a coma corrector in a newtonian as long as f/8 to accomplish it.
I guess some people never look at the edge.
7) the f/ratio of the scope. The coma free zone in the middle diminishes VERY rapidly below f/6. To wit:
8) the quality of the optics in a scope. If you never see tiny point-like star images ever, then coma is the least of your worries.
At f/4, the star images at the edge of the field of a 31mm Nagler are close to 200µ in diameter, while the Airy disc is 6.4µ in diameter.
If I had a scope that bad, I'd have the mirror refigured. Some of the reasons above may be determinative of whether coma is obtrusive.
Edited by Starman1, 18 May 2021 - 02:50 PM.