[ATM] Pyrex vs Plate

[bobmay at nethere.com (Bob May)]

That can be a big argument if it gets out of hand.
The thing is that pyrex has a coefficient of expansion that is a lot smaller
than that of plate glass and, as such, won't change its figure when the
temperature changes.  This thus makes the pyrex a lot nicer when the temp
changes for whatever reasons.  The thing tho is that you have to compare two
mirrors of the same size and thickness when doing this comparison.  Use a
thinner piece of glass for the plate glass mirror and the comparisons can
actually end up the other way wit the plate glass coming to a satisfactory
state of figure in a much shorter timeframe than the pyrex one and thus work
better than the pyrex one.
I'll note also that the 200" Hale telescope is the first large telescope
mirror that got built.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
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