Re: ATM Cassegrainian coudé: weird image at focal plane

["Bob May" <bobmay@nethere.com>]

If you're getting an image of the moon nicely (as you have stated) on a
piece of paper and adding in the tertiary mirror to that setup won't produce
a proper image again, then the tertiary mirror has to be at fault.  That
mirror at 45 degrees does nothing (or should do nothing) to the light cone
but change its direction, most assuredly not changing the focal length or
other condition of the light cone.
In any optical system, one of the accepted things is that a flat surface can
be inserted anywhere and the only restrictions to the optical system is that
the flat surface will obstruct the light going from one place to another.
Needless to say, the requirement for the flat surface is that it is indeed
flat and not some gentle curve.  Even if the surface is a smooth spherical
surface, the end result is astigmatism from the curvature acting on the
bouncing of the light cone.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net