[ATM] Re: Second Thoughts on Mirror Support

[peter at kitgear.com (Peter)]

Try it.

Test a good mirror, mount it using RTV and measure it at differing ambient
temperatures to see what you get. You'll find that the CTE of glass and
steel are not as close as you think.

Then you can look up the SPIE course on Introduction to Optomechanical
Design using kinematic mounts and review your design to see if you'll do it
the same again or not.

Peter

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of Jeff Anderson-Lee
Sent: Wednesday, February 04, 2004 8:50 PM
To: The Amateur Telescope Makers List
Subject: Re: [ATM] Re: Second Thoughts on Mirror Support


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

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