[ATM] Re: Second Thoughts on Mirror Support

[aart at insightbb.com (Aart Olsen)]

Stuart Hutchins wrote:
> Whiffle trees support the back in a uniform manner, mimicking flotation in a
> liquid.

I'm sure Stuart knows this already but it should be pointed out that the 
ideal of liquid-like flotation is a misnomer and is NOT the goal of a 
good cell.  A telescope mirror floating in a pool of mercury is actually 
strained: it would have too much support in the center where the glass 
is thin and too little support at the edge where the glass is thick.  A 
good whiffletree mirror cell provides support proportional to the glass 
thickness, which is not uniform.  Tools such as PLOP can design an even 
better cell by intentionally undersupporting the mirror center (in the 
secondary shadow) so that the whiffletree can support the surrounding 
glass even better.  This is the opposite of what liquid flotation would do.

-- 
Aart Olsen
aart@insightbb.com