Ricks article on Coma

Thanks for a good article Rick.
We appreciate your easy reading straight forward approach.
I would ask one thing though. It was stated by Vladimir in another post that incorect collimation does not actually effect Coma, but only increases it asymmetrically.
Do you agree with that statement? You said to be sure that the scope was properly collimated.
I for one see a big change in my SCT over the entire FOV if it is not in collimation. Even the sweet spot in the center is effected. It also seems to me that the Newts are effected as well. I am just seeing things?
Floyd

Coma is an inherent property of a paraboloidal mirror. If the optics are properly lined up, coma is symmetrical about the center of the field. If things are not properly lined up, it is asymmetrical. If they are not too far off, somewhere in the field is the coma free region of the mirror.

In many cases, astigmatism in the eyepiece overwhelms the coma, and often it is this that amateur notices. Once the optics are misaligned, the center of the eyepiece and region of best performance no longer matches the coma free region of the field, so things may not look very good anywhere in the field. It depends on how badly the telescope's optics are misaligned and how wide a field the eyepiece covers without significant aberration.

Clear skies, Alan

Floyd, the thread in which I said that miscollimation doesn't effect coma
was for Newtonians. Flat reflecting surface (the diagonal) doesn't induce any aberrations; it can only cause decentering of the primary's image in the eyepiece. This results in the coma-free portion of the image being shifted away from the center of the view, which now contains comatic portion of primary's image (that is, if miscollimation image shift is sufficient to bring in noticeable coma). At the same time, the coma free-portion becomes a "subject" of eyepiece's off-axis astigmatism (to the extent determined by the eyepiece type), so that no part of the field has really good definition. This grows in significance with the increase in magnification. At low magnification, moderate miscollimation has little effect.

It's different story with SCTs. Miscolimation here *creates* coma (some astigmatism too, but insignificant in comparison), even if the system is aplantic (coma-free) when properly collimated. And it is rather efficient in it.
So, while with fast near-perfectly collimated Newtonians coma still hangs in at medium-to-low powers in the outer field, with SCTs you can go from no visible coma at all to a very visible coma caused by miscollimation. There's nothing contradicting in your experience.

Almost missed to mention - Rick, good article. But if you want to get a real chance to win, change the subject. Optical theory, even in pre-digested form, is never very popular meal on the menu
:S