RE: [APML] Photographic test of Meade 8" f/4 Schmidt-Newton

[Alen Koebel <alen_koebel@yahoo.ca>]

Walter Koprolin wrote:

> The use of a spherical mirror makes the optical
> system coma-free, one of the advantages of a 
> Schmidt camera. 

That would be true if the corrector were at the 
radius of curvature, as it is in a Schmidt camera.
With the corrector of a Schmidt-Newtonian actually
within than the focal distance, the system is 
unfortunately neither free of coma nor free of
spherical aberration (Schmidt cameras are free of
both), according to Rutten and van Venrooij. But
the coma is much better than with a standard 
Newtonian design. 

> The problem here is the curved focal plane, so the
> edges of the flat film plane will be slightly 
> out-of-focus and stars there distorted radially.

Both  problems are there. I can see evidence of 
both in my slides, although they are not severe.

> "Creative focusing", i.e. focusing not at the 
> center of the FOV but somewhere further out 
> will also reduce the distortion at the edges, 
> but will create slightly bigger stars at the 
> center.

That's the usual compromise. 

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