RE: [ATM] Cell induced astigmatism

["lou krajci" <loukrajci@comcast.net>]

>From: "Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>

>I put up a little article about the astimatism you may introduce when you
>glue your mirror to its flotation cell.

>See:  http://www.homepages.hetnet.nl/~artm/atm/resource/deformation.htm

Thank you for the analysis!  Some points:

1.  You mention P-V surface deformation, but not RMS.  Could you tell us
what RMS values resulted from your analysis?

2.  Based on some thoughts I've had about the next 6 point mirror cell I may
build:  In your fig. 1B (Cantilever dimensions)...if you bend the middle of
the cantilever bar so that it comes closer to the back surface of the
mirror...you can reduce the dimension from 7.5mm to something smaller.  This
will help reduce the force couple when observing near horizon.

3.  I have a 16 inch diameter, Pyrex mirror, that's 1.5 inches thick, on a
six point support.  It's very similar to the design you describe in your
analysis.  I have used it for five years.  I have star tested it, and so
have some other folks.  So far...nobody has mentioned problems at the
eyepiece that might come from mirror cell induced image distortion.

At the end of your page you state:  "...the presented results discourage
gluing the mirror to its cell."

Point number 3 above seems to indicate that gluing the mirror to the cell
can work...at least in the cases I've dealt with (to include a ten inch, 1.3
inch thick mirror on a four point support).

I hope this helps.

Tom Krajci
Albuquerque, New Mexico
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