Re: ATM mirror cell suport

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

The effects of sitting a mirror on a stand vertically have been seen a fair
bit by the people that have been making thin mirrors.  One of the things
that you do have to remember tho is that the astigmitism is in the vertical
dimension and the Foucault test is normally done across the mirror in the
horizontal direction (although I have seen testers that do it in the
vertical direction, they are rare) and thus the astigmitism is usually not
really seen except when the star test is done on an artificial star.
One of the things that you do have to remember is that the thick mirrors
that used to be made were quite tolerant of the support system that is
typically used (two posts sticking out from the back board) for both the
reason above and the fact that the error is going to be very small in those
mirrors as they are nice and rigid.
Another reason is that when you do the Couder mask system of the Foucault
test, you never see anything very far from the horizontal axis and thus the
astigimitism is hard to even see as you don't see any of that vertical error
from the support.
Now that people are starting to regularly grind very large thin mirrors, the
problem of supporting the mirror for testing is starting to become reality
and thus the modifications that are starting to happen to the way that the
Foucault test is set up.  It probably won't be too long before it will be a
requirement for a 2 story tall building interior (or more) will be needed in
order to be able to test a mirror of the meter size.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net