Re: [ATM] Large Schmidt Camera?

["vladimir sacek" <vla@toast.net>]

David Sleeter wrote:

> I have a 16-1/2" dia. F-2.5 spherical Cassegrain-style (hole in the 
> middle)
> mirror, and I'm wondering if there are any off-the-shelf combinations of
> field flattener and CCD camera with which it could easily be used as a
> Schmidt camera?

It won't work like Schmidt camera, but maybe something useable could
be done with a split reversed meniscus. I played with it a little before.
It can correct spherical aberration and astigmatism of fast spheres quite
well, with acceptable chromatism induced.

I just threw in two BK7 lenses of identical radii, plano-concave first and
plano-convex following (both facing the mirror with the curved side),
with an arbitrary mirror and lens separation, tweaked the radii until the
s.a. was well corrected, and the violet blur came at 40 microns in
diameter. Not too bad for the first random try. It could certainly be 
better,
maybe significantly, with a radii/separations/indici optimization.

The problem is that the coma is about twice that of the mirror alone
(which is a lot already). If it could be reduced to that of the mirror 
alone,
a Paracorr could enter the play. Seems worth investigating.

Vlad 

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