Re: ATM Why gregorian?

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

I am working on what I call a Schmidt Gregorian.  The design is using a
Schmidt corrector plate to convert the F1.95 primary to the parabolic
condition and my then even do a secondary corrector plate just for the fun
of it.  If I do it this way, all of the surfaces are ground and polished as
spheres, making the whole thing a supposedly easy scope to build(!) although
assembly will be a bit more difficult with all of the parts.
As to the Gregorian, the image is right side up, all of the surfaces are
concave so testing is straightforward and none of the surfaces are more than
a parabola.
The scopes are going to be longer than an equivalent Cass design but only by
a little bit.  The secondary is on the other side of the primary focal point
by about the same distance so that may mean about 3" or so for a typical
sized scope for the amateur.
FWIW, I've just put the corrector plate on the vacuum pan and am looking to
see if it is holding vacuum which it is seeming to do so.  I'll be casting a
tool to do the 5 micron grinding and then polishing of the two surfaces (I'm
putting correction on both surfaces to keep the vacuum requirements down and
allow a thicker piece of glass for the plate).  After that comes the boring
of the center hole for the mounting of the secondary and finally doing a
secondary for the scope.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net