Re: [ATM] Offner layout question - links-corrected

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

Ric Rokosz wrote:

> Anyways,the knife edge for the tester was to the side of the Offner
> assembly.I had always assumed you need to view the return beam throught
> the lenses as in the Ross.Wonder if this is O.K. ? Would imagine the
> offset would introduce some (lot?) of error.

It doesn't really matter whether the wavefront is corrected before or after
mirror. As for the offset, it does look too large, I'd say about 3/4 degree
for the 30" mirror setup. Since astigmatism doesn't change with object
distance, you can use W=(Da^2)/8F for the wavefront error, "a" being the
offset angle in degrees. For a 30" F/4 it would come to more than 7 waves PV
wavefront error of astigmatism (use the infinity-object mirror F# in the
formula).

Coma is zero for object at the r.o.c.  for mirror with the stop at the 
surface. It changes with (m+1)/(m-1), "m" being the mirror magnification 
m=-f/(o-f), with "f" the focal length and "o" the object distance.

Vlad
www.telescope-optics.net

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