Re: ATM schmidt-newton

[Bratislav Curcic <epabcc@epa.ericsson.se>]

Achim Schaller wrote 

> As I know from real Schmidt-cameras, the corrector
> plate has to be at the radius of curvature (and not close to the focal
> plane like in the Comet Catcher). So if Id place it at the RC, would
> image quality be improved? 

Er - yes and no. You will remove residual coma, but accentuate curvature
of the field. A proper ray trace will tell you the net gain. There is one
obvious loss - you would have to introduce a spider (and diffraction 
effects from it). Some people actually prefer cross shaped bright stars.

> If I did, would I get more vignetting (because of the longer tube)?

Yes. Primary in Schmidt must be substantially larger.

> I have another question concerning field curvature. Would the field be
> flattened using a field flattener like those available for refractors?

No, it would get much worse as refractors have field typically curved 
CONCAVE to the sky while Schmidt has field curved CONVEX to the sky.
You can make simple field flattener (Piazi-Smyth lens), but it has to
be very close to the film, i.e. forget SLRs.

Bratislav