No, it affects all Newtonians and most other types of telescopes to some degree as well. A system with no coma is called aplanatic.
> My question is: Is coma inherent in the
> Newtonian relector's design or is it a figuring error in the
> primary mirror?
It is inherent. The simplest design with no coma has two non-spherical mirrors. (like the HST was supposed to be)
> If I were to grind a perfect mirror at any
> f ratio would I still have coma?
For a telescope with a single curved mirror, Yes you would. Although it is much less at slower focal ratios.
> Does coma tend to occur in
> large, fast Newtonians because their mirrors are harder to
> figure than long focal length mirrors?
No, it is inherent that faster focal ratio parabolas have more comma than lower focal ratio parabolas. The diameter of the coma free field (1/4 wave of path length error peak due to coma) is a function of the cube of the focal ratio. An f/4.5 Newtonian has a coma-free field about 1mm in diameter by this measure as I recall. There was an article in Sky and Telescope about calculating this using path differences a while back.
> Just curious (and hoping to someday regrind my 10" to get
> rid of that nasty coma!)
Regrind it to a slower focal ratio is the only thing that will help.
Coma correctors, and the Pretoria eyepiece have been mentioned for helping with the edge of field sharpness lost to coma. However, there is probably something else you can do at the eyepiece end of your scope which will help. Just get a really good eyepiece. In most fast Newtonians, that do not have really great eyepieces, most of the blur at the edge of the field is actually caused by astigmatism in the eyepiece, and not coma. Most eyepieces have a lot of astigmatism when used at f/5 and below. Correcting for astigmatism down to about f/4 was one of the great breakthroughs by Al Nagler in the design of the Nagler eyepieces and was one of the primary reasons the field of view can be so wide. Eyepiece astigmatism increases rapidly with increasing angular aparent field of view (I think it is quadratic). Coma only increases linearly with angular field of view so it is not that much worse 40 degrees off axis than it was 30 degrees off axis.
There was a visual comparison review/test published a while back (I think it was in ATMJ) between a Nagler and a Pretoria. The result was basicly, it looks better through the Nagler ie. it is more important to correct the eyepiece astigmatism than the primary coma. I think that it stands to reason that if Pretorias were that great, then they would have sold a lot more of them, and they would still be available...
Assuming your mirror is ok, I would suggest borrowing and trying a Nagler eyepiece sometime before regrinding your mirror to really evaluate how bad that coma is to you.
Chuck