From: George Anderson 
To: Richard Schwartz , atm 
Reply-To: George Anderson 


Thanks Richard

I realized about two hours after I posted that 20 seconds of thought on my
part before posting would have answered my own question. It took my 2 year
old son to wake me up at 1:00am (he had a cold) and I gave it a little more
thought. Also I believe (I'll have to check R&V) that the focal plane
in a newt is reversed to a schmidt. Lesson learned:Never try being really
creative when  you are exhausted. You'll overlook the painfully obvious.

George Anderson
Montreal Canada

Clear skies and good health

Richard Schwartz wrote:
>
> how big is the f/4 mirror?   The important thing here is called "petzval
> curvature"; that is the curve of the focal plane.   Find the sag of your
> focal plane from the curve, and divide that by 4 (your f-number) to estimate
> the blur due to field curve.
>
> Coma is a very different problem.   To get rid of it, you need a second
> optical surface as in the R-C telescope.  To reduce coma, use a longer focal
> length mirror.  Generally, coma is given by
>
> K=(3/16)*u / f^2
>
> u is the distance off axis (mm in the focal plane, or angle in the sky)
> f is the f-number of the mirror
> K is the coma in the same units as u (linear in the focal plane, or angle in
> the sky)
>
> . . . Richard
>
> ----- Original Message -----
> From: "George Anderson" 
> To: "atm" 
> Sent: Thursday, April 24, 2003 8:24 PM
> Subject: ATM Coma and a curved focal plane
>
> >
> > A question about photographing with a fast (f/4) newt...
> > If the film surface was curved like a film holder for a schmidt camera
> > would this eliminate coma or would I just have clean sharply focused
> > coma?
> >
> > George Anderson
> > Montreal Canada
> >
> > Clear skies and good health
> >
> >

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