Re: ATM Ritchey-Chretien

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Matt wrote : 

> (Ritchey-Chretien) usually has a strongly curved focal plane

and Tim Puckett wrote also :

> The field is virtually flat across the entire filed and the star 
> images elliptical

The curvature of the field depends on the difference in radii between
primary and secondary mirrors, hence on secondary magnification. There
_are_ flat field R-C systems, but not many around, as they have very
fast primary and a huge central obstruction (the secondary
magnification has to be close to 2, which results with more than 50%
central obstruction).  So, I doubt that Tim's 'scope is "virtually flat
across the entire field", as secondary seems way too small.
Although possible in principle, these configurations are usually
photographic only systems.

The other alternative is to use focal plane correctors, and it is possible
to simulatneously correct astigmatism and curvature of the field in R-C
configurations (that's what pros do).

Bratislav