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Re: [ATM] 40" mirror

Quoting "Jerry B. Hillman" <JBHillman@ev1.net>:

> Hi Jeff,
>
>> Also consider a thinner mirror.   Four inches of glass will almost never
>> come to equilibrium.
>
> I thought that is why pyrex is the substrate of choice.  Low thermal
> expansion co-effcient.
>

Heat retention is a seprate issue.  I think Mel once said something like a one
inch mirror will take an hour to cool, a 2 inch about 3 and a 3 inch mirror
will take longer than the observing session.  That is why I suggested you
search the archives for this in my first post.  I probably have the times
wrong, but the idea I beleive to be an accurate reflection of his post.

I think active cooling design may have improved since then but 4 inches 
is still
one heck of a chunk of glass to cool.

Hermit, holed up in Youngstown, Ohio
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