[ATM] Re: Second Thoughts on Mirror Support

[mikell at optonline.net (Michael Lindner)]

On Wednesday 04 February 2004 09:01 pm, Jeff Anderson-Lee wrote:
> Out of curiosity, what's the formula used to derive 0.007" from the
> dimensions and the Compressive Yield Strength?

FL/EA where F is force, L is length of column (in this case thickness of pad) 
E is modulus of elasticity and A is surface area of pad, The formula is in 
Kreige/Berry's book "The Dobsonian Telescope", although it isn't applied to 
RTV pads in there. ;-)

FWIW I find nothing wrong with RTVing a mirror to supports, and have made all 
my scopes this way. However, 9 pads are not the same as 9 floating points. 
The whole purpose of a flotation cell is so that the forces are evenly 
distributed among the support points, even if the cell sags or warps. RTV 
pads may compress, but they will still transmit force through to the mirror.

The difference might be negligible from pad to pad on a bar or triangle (or 
near zero if they're the same distance from the single support point, so it 
sags the same at each mirror attachment point), but across the entire length 
of the cell, itself supported at only 3 points (for collimation), uneven 
sagging may transmit enough force to be detrimental. Also, unlike fine 
annealed glass, metal may distort over time enough to create uneven force on 
the back of the mirror.

-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
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