Re: ATM Mirror surface accuracy

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

One of the things that I find fascinating is how often people tend to paint
things in absolutes - there's only 1 set of curves for a Cass is one of
them.  The R-C design is only a particular point on the shapes of the
primary and secondary of a Cassegrain System.  Moving a little to one side
or the other of a particular point of the the curve set will modify the
abberations only a little bit compared to what is designed.  Thus, it's not
going to matter too much if the secondary is reshaped from the original
design spec as it's usually a fair bit smaller (there are telescope designs
in which the secondary is actually larger than the primary!) than the
primary and thus is a lot easier to move about and change the surface of.
All this is, of course, academic to the problem of surface shape of the
secondary of a Newtonian type design as different things are happening
between the two types of scopes.  In the Newtonian design, the secondary's
sole importance is to change the direction of the light coming to a focus
and thus, it needs to be a flat surface as accurately as possible.
Bob May
http://nav.to/bobmay
bobmay@nethere.com