Re: ATM : what is Coma

["Michael P. Lindner" <mikel@att.net>]

Donovan Buck wrote:
> 
> How does a coma corrector work 
> and where in the light path does it fit. How well do these devices
> work?

You fix coma by essentially introducing "negative" coma. That can be
done in many ways, and at many possible places in the light path. A coma
corrector must be matched to the whole optical system to exactly cancel
coma.

I'm assuming you mean commercial coma correctors, which reduce coma to
some extent, but do not eliminate it. They work in a limited range of
focal ratios (the Paracorr works best in the f/4 to f/5 range) and
wavelengths (i.e. the Paracorr is for visual observing - not
photographic). As for how well they work, I know people who swear by
them, and people who dislike them (just like barlows and various
eyepiece designs, it's all a matter of opinion).

Ashish, Balaya (IE10) wrote:
> 
> Was just wondering, if one
> surface cannot be made free, how about figuring the secondary

The problem with a diagonal secondary is that the light is striking it
at an angle, so any curve you put on it will introduce astigmatism. If
you mean something like a cassegrain secondary I guess it's possible,
but I don't have the slightest idea how.

--
Mike Lindner