> >That's because what you are seeing isn't coma.
>
> Hmmmm. OK. I'm willing to be (coma) corrected.
>
> Can you elaborate?
>
The reason why I'm sure that there isn't less coma in
the NGT12 vs the Coulter at the same f/ratio is that
coma is simply not improved by mirror figure, it is inherent
in fast paraboloids.
The vast majority of the aberration that people
think of as "coma" is really astigmatism of the eyepiece.
Coma is more important in imaging and photography
where the longer exposure can make the relatively
faint comatic tail more obvious. In some circumstances
it is visible visually, especially when using wide field/
long focal length eyepieces, but near the center of the
field, it isn't all that objectionable.
Better eyepieces (eg Panoptics vs. Erfles) have better
off-axis correction, and can handle the wide light cone
of an f/4.5 with less astigmatism. See Rutten and
Van Venrooij, "Telescope Optics: Evaluation and Design"
for more info and a better explanation than I can give
right now.
There are a couple schools of thought on coma correctors.
Some people think you should skip the corrector (specifically
the TV Paracorr) until you've upgraded your eyepieces to
the point where you really are limited by coma. Others
find that the field flattening of the Paracorr makes observing
less fatiguing (because the eye needn't accommodate the
curved field as much). Personally, my eyes work well enough
that I don't find the field curvature objectionable, and I'd
rather spend the money that I don' t have on EP's as opposed
to correctors, but I'll probably change my tune when I hit 40 ;-).
-- archer