Re: Gregorian Secondary - Discrepancy in ATM1 ?

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

The Gregorian secondary has a real focus point (its concave so light hitting
the surface will focus to a real point) while the other Cass designs all
have secondaries with an imaginary focus point (the surfaces are convex and
thus the reverse of the reflection leads to a focus point which is behind
the surface).
This means that the concave Gregorian secondary is tested just like the
primary is while the convex secondaries need to have auxiliary optical
surfaces to make the light appear to come from the imaginary focus.  The
Hindle Sphere does this function by reflecting the light back to the source
and you then test the return for errors.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net