[ATM] Flex / Potato Chip / Astigmatism question

[flex at commspeed.net (Bill Kelley)]

Ian,
I forwarded your post to Alan Adler. He may offer some comment.
Bill

----- Original Message ----- 
From: "Ian McKernan" <mckernan@murdoch.edu.au>
To: <atm@atmlist.net>
Sent: Sunday, February 01, 2004 10:41 PM
Subject: [ATM] Flex / Potato Chip / Astigmatism question


> Hi all!  Am working on an 8" flex mirror at the moment, but was playing
> around with FLEX.exe and trying some different numbers.  Dreaming of  a 20
> inch flexed in the far future.
>
> Anyhow, according to FLEX.exe, a 20 inch diam f6.0 sphere, with a flat
> back, 0.95" thick (since i'd be lucky to even find 1" thick glass here in
> W.A.) will flex to a 1/16.5 wave-front error parabola.  This seems pretty
> good to me, and useable if it can be fabricated. There are a couple of
guys
> locally who have glass that they are going to do 1" thick 20" mirrors out
of.
>
> Problem is that one of the outputs is "Gravity Effect" which = 274% for
> this mirror.  According to the program documentation:
>
> " MAX GRAVITY EFFECT.  THIS IS THE EFFECT OF CHANGING
>   ALTITUDE.  100% GRAVITY EFFECT WOULD JUST REACH THE
>   THRESHOLD OF VISIBLE ASTIGMATISM AT ZERO DEGREES ALTITUDE.
>   GRAVITY EFFECT IS FACTORED BY THE COSINE OF ALTITUDE.
>   THUS THERE IS ZERO ASTIGMATIC GRAVITY EFFECT AT ZENITH."
>
> I assume that this is a "potato chip" effect where the mirror tries to
fold
> over under its own weight.
>
> My query is, does this happen with normally mounted, large thin mirrors,
or
> is it a problem peculiar to flexed mirrors??
>
> I would think that with a wide annular puller mounted on the back of the
> mirror, and tension flexing the mirror, that this would be less of a
> problem than for a mirror mounted in a free floating cell.  Would be
> interesting to hear from people who have made inch thick large mirrors and
> used them, (particularly James Lerch, and his mate Ron with the 22" 3/4
> thick mirror?) or from anyone out there who is into materials modelling.
>
> Seeyahs!
>
>
>
> *********************************************************************
> Ian McKernan
> Plant Biology Technician
>
>          c/o Murdoch University
>          Division of Science and Engineering
>          School of Biological Sciences and Biotechnology
>          South St Murdoch  6150
>          Western Australia
>          tel: + 61-08-9360 2206
>          fax: + 61-08-9360 6303
>
> Don't let the Dogma Bite!!
>
> *********************************************************************
>
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