Re: [ATM] Test stand astig., edge support

[Dale Eason <atmpob@yahoo.com>]

I have been trying to measure test stand induced
errors using Steve Koehler's OMI Torus 18 X 2 pyrex
mirror and my Bath interferometer.  I have been able
to generate much astig with many methods of lateral
support. Up to 1/3 wave pv on the surface.  The
magnitude is much higher than I thought would be
possible with this thickness ratio, but I have little
experience in this regard.

I have tried wire and cloth slings, 90 deg and less
pegs, mirror vertical, mirror tilted back 10 deg with
three back support points....

The biggest problem to date is that I cannot control
the amount of induced astig. It changes with each
adjustment of the mirror.  Tiny changes in sling
position, or the way the mirror is balanced on pegs
changes the astig dramatically when the mirror is
vertical.  I don't have the resources to tilt it back
more than 10 deg and that is not enough to stabilize
the terms. Even when tilted back at 10 deg the primary
astig magnitude is close to 1/4 wave pv on the
surface.

The second problem is that I don't know the actual
astig in the mirror.  However it should be small it is
a very good mirror and star tests good as well.  It
would be nice to have a sphere of known quality.

If any are interested I can give more details and
upload my current results on the atm_free or
yahoo.interferometry group where I find it easier to
upload and view photos and data than on this list. 
There is a couple of megs of contour maps and Zernike
terms derived from my interferometer setup. 


Dale Eason


		
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