Re: ATM cassagrainian secondary

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

The only version of the Cass system you can really build this way is the
Dall-Kirkham without going to some real expenses with the secondary
construction.  Spheres are easy to do as the interfrometer looks at straight
lines in its output.  Any other shape needs to be calculated for the
curvature of those lines and then further work done on a distorted lap in
order to get the results desired.  There is a reason why professionally
produced parabolic mirrors are only 1/4 wave on the surface and that is the
time to get the shape any better than that.  Most of us would consider a
mirror of that quality to be the low end of Coulter's work when he was
spitting out the mass of telescopes he did at a high rate.  Most of us
aren't happy with our work until it's at least 1/10 wave on the wavefront
(1/20th wave on the surface!) or better!
If the professional can do a test quickly and get a spherical null, he'll do
it but still he'll charge a lot for the additional work just to get the odd
shape into the glass.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net