[ATM] Re: Second Thoughts on Mirror Support

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

Michael Lindner wrote:

>>>Does anyone know how compute how much compression a 25mm round by 3mm
>>>thick RTV pad would experience under a 0.306kg load?
>>>
>>From http://www.matweb.com/SpecificMaterial.asp?bassnum=O5200
>Compressive Yield Strength of silicone is 4 MPa or 580 psi.
>
>Using english units (sorry, rest of the world), a 1"x1/8" cylinder holding a 
>2/3 pound mass would be compressed about 0.007". Empirical confirmation of 
>this analysis is left as an exercise to the reader. ;-)
>
If that's the case, then a 1/8th in steel plate should be stiff enough
(by more than an order of magnitude) to hold a 16in by 1+5/8 in f/4
pyrex mirror with just 3 bolts and 9 blobs of RTV.  No messy levers
or ball joints.  You still need to put the blobs in the right places though.

I re-ran the numbers on the 1/8th inch plate and the P-V is really 6.8e-3mm
or 2.7e-4 inches. (I think I must have had a residual refocus factor in 
Plop
the last time I ran the calculation.) For a 1/8th inch steel torus 
12.65in in
diameter with a central 4.59in hole, the P-V is 5.1e-3mm or 2.0e-4 inches.
Thus you don't need a full plate behind the mirror if you want to reduce the
thermal mass.

Jeff Anderson-Lee