[ATM] If it's in a supremely stiff cell, that does not twist,why would a thin mirror sag?

[jonah at eecs.berkeley.edu (Jeff Anderson-Lee)]

artbianconi@blast.net wrote:

>No matter how I try I cannot fathom why a thin mirror would not maintain its 
>parabolic surface if held by a uniformly planar cell made of of carbon fiber.
>  
>
Did you grind and polish the back of your mirror into a 10th wave flat?  
Will you do the same with the carbon fiber?   Otherwise, how you you 
know they are "uniformly planer"?   One problem comes in trying to make 
it stiff AND flat at the same time.

Other kinds of cells (e.g. wiffle tree) work because we don't NEED the 
cell or the (back of the) mirror to be made to 1/10th wave tolerances 
(in terms of position and size of parts) in order to get a 1/10th wave 
support on the mirror.
 

>If it be true that the sagging issue begins with using a thin mirror, instead of a 
>thick one and goes away to a major degree when the mirror is made thick 
>again, then any properly designed supplemental support made from suitable 
>materials, should restore the correct geometry and prevent the distortion.
>  
>
Does carbon fiber have the exact same CTE (coefficient of thermal 
expansion) as your mirror glass?  If not, then as the mirror and support 
heat and cool,  they will expand and contract at uneven rates and your 
custom contoured cell will no longer match the mirror.  Even if they 
were both sufficiently flat (e.g. 1/10th wave) there would be a problem 
of stiction between the mirror and support putting sideways pressures on 
the glass.

>There seems to be a great deal of resistance to this logic.
>
>I don't understand.
>  
>
There is a great deal of other logic that is resistant to this design.  
But, of course, you are always free to try it and report back! Were just 
suggesting that you don't bother--but whether or not you try it is 
totally up to you.

Jeff Anderson-Lee
Sacramento, CA