[ATM] what causes coma?

[mdholm at telerama.com (Mark Holm)]

I think Donald Good's explanation is really a description of field 
curvature, an abberation also present in many reflecting telescope 
designs.  With field curvature, if you could warp your detector to fit 
the curvature (possible with photographic film and plates if the 
curvature isn't too strong), the abberation would vanish.  With coma, 
the abberation would still be there, even with field curvature 
corrected, no?  Schmidt cameras have no coma, and, I think, no 
astigmatism, and most or all of the spherical abberation canceled by the 
corrector plate, but, at least in the "pure" design, there is major 
field curvature.

The explanation at the OMI web site is probably technically correct, but 
I find it somewhat incomplete because, the way it describes coma makes 
it seem that coma could arise from a parabolic or spherical mirror.  I 
think spheres are coma free (if I am remembering what I have read 
correctly) but of course spheres have lots of spherical abberation.

Maybe I am just not understanding what has been written.  Anybody else 
want to try an explanation that doesn't involve too much math?

Mark Holm
mdholm@telerama.com