[ATM] Flex / Potato Chip / Astigmatism question

[mckernan at murdoch.edu.au (Ian McKernan)]

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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