Re: ATM grinding first mirror

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

Let me put things into perspective!
We're mostly talking about really excellent adjudging of mirror, often well
under 1/20th of a wave of error.
To give you a reference, several years ago, Peter Ceravolo did up 4 6"
mirrors that had known errors of shape of 1/2, 1/4, 1/8 and 1/16th wave of
error on them.  He then took them to several of the big astronomy shows
where he had people grade them for quality.  Only a few people could detect
the difference between a 1/8th and 1/16th wave error telescopes and then
only when the skies were excellent.  Some people couldn't tell the
difference between them.  That's the two ends of the spectrum for being able
to see something odd in any of the scopes.
As a result of those tests, a 1/4 wave error for a first mirror is decent
quality for most individuals.  Some would want to do better and others don't
need to do as well - it all depends upon how critical you are of your
mirror.
As to the testing, we are mostly describing the errors that happen with the
1/6th wave error type stuff and above for quality.  For thin mirrors (ratios
of greater than about 1:15 or so) there is another class of problem that
happens in that the mirror is unable to properly support itself on the
mirror stand and this is something that is going to be true no matter how
big you really get.  This is a long way from getting a simple 1/4 wave
quality.
I might note that there are interfrometer tests that can be done on a mirror
without too much optics in the path and the big problem there is getting
somebody to actually do the setups with simple optics and report back to the
rest of the world about how to do it.
I might also note that the Ronchi Test is an excellent test for determining
whether the mirror has a good shape or not as this is about half the problem
of getting a good surface.

Bob May
http://nav.to/bobmay
bobmay@nethere.com