[ATM] Re: Cell induced astigmatism

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

Hi Tom,

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

No, but I would estimate about a factor 5 less, considering the regularity
of the deformation.

> 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.

That's something I've thought of myself, but considering the required beam
stiffness, silicon pad thickness and diameter of axis i believe you would
end up with several mm in any case. Maybe this way you can decrease the
effect by half or so?

> 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.

This is what worries me most. Earlier discussions also indicated that the
astigmatism was not seen. I wouldn't know, because my mirror is supported on
teflon points. However I do recall astigmatism problems Jan van Gastel had
with his (initially) glued 20" mirror.

The incentive to start this analysis was to check my gut-feeling that the
force couple should yield a visible effect. For this reason I requested a
(student) copy of MSC Nastran, which I did not get. Instead, the very
helpful dutch MSC representative offered to do the model for me, and
returned the results as shown in the article. Hopefully he will do the
follow-up as well :-).

If anybody wants to experiment any further, please let me know and I send
the Nastran model / mesh file.

/Arjan

_______________________________________________
ATM mailing list http://www.atmlist.net/