Re: coma, coma, coma

Mark VandeWettering (markv@tick)
Mon, 1 May 1995 10:58:52 -0700

On May 1, 9:12am, Richard Schiek wrote: > Subject: coma, coma, coma
>
>
> I've got a question about coma in Newtonian reflectors.
> From what I've read and seen (in my 10in. f4.6) coma is an
> optical problem effecting the edge of the field in fast
> Newtonians. My question is: Is coma inherent in the
> Newtonian relector's design or is it a figuring error in the
> primary mirror? If I were to grind a perfect mirror at any
> f ratio would I still have coma? Does coma tend to occur in
> large, fast Newtonians because their mirrors are harder to
> figure than long focal length mirrors?

All parabolic mirrors suffer from coma. The magnitude of the coma goes up rapidly with decreasing f/number, I believe quadratically. In other words, if you half the f/number, you quadruple the coma.

Large fast mirrors are harder for two principal reasons: 1. They are bigger, so more glass has to be removed. 2. Because the light cone converges at focus at a steeper angle, smaller deviations in the surface get magnified more. (I could go in greater detail, but I would need a blackboard...)

By the way, what you are probably seeing at the edge of the field is not coma of the primary, but astigmatism of the eyepiece. Try focussing in and out, if the blurs go radially on one side and tangentially on the other, you have got astigmatism, not coma.

Mark

> Just curious (and hoping to someday regrind my 10" to get
> rid of that nasty coma!)
>
> Thanks,
>
> Rich
> richard@chemeng.stanford.edu
>
>
>-- End of excerpt from Richard Schiek

--
Mark T. VandeWettering                  Telescope Information (and more)
Email: <markv@pixar.com>                http://webspace.com/markv/
       <markv@webspace.com>             Clear Skies!