Re: ATM Why?

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

Your HTML is on again Richard!  This time it's dark green with black
lettering.
You have to remember that most of the telescope makers are barely at the
point where they can do a surface other than a sphere.  There's a lot of
writings and software on making the parabola from that sphere but there's
actually very little on making surfaces other than that.  Making things a
bit more complex is the problem of having two different methods, e and SC,
for defining that surface.
Finally, the idea of testing a convex surface is also a real problem as this
requires (for the best results) more optical surfaces than just the simple
Foucault tester (look at all the questions that every newbie seems to come
up with on that subject) and mirror stand.  I build and occasionally sell a
tester and just that little bit of technology bought rather than built seems
to help (they now know that they have something that is a known good item
that somebody has made work!) a lot of those who haven't done it before.
For somebody like that to set up the Hindle test is something beyond what
they have gotten to with the basic test.  I might also note that I've seen
some complex processes for setting up the tester have been done to the same
end result that is done with a simple pointing of the mirror to the tester
so that the returned light goes to the correct place.  For anybody that
thinks that the tester needs to point exactly at the mirror, I'd suggest
that they do the math needed to see how much an error of measurement is
obtained when the tester is pointing 5 or 10 deg. off of the mirror.  The
results may suprise you!
Also, Richard, I'm starting to figure that glass of yours.  I got stupid and
went off to a quick 2.5x of paraboloid because I didn't watch how much time
I had put on the figuring.  I'm now going back to a sphere again.

Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net