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Originally Posted by ausastronomer
Thats not quite correct either...
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That was an interesting post. I had forgotten to consider things other than thermal properties.
I found a site that has the properties of mirror substrates and the steady and transient thermal distortion characteristics of BK7 are between 1.5 and 2 times as good as plate glass. But just stiffness is not much higher (73.1 vs 81 GPa).
http://www.minerals.sk.ca/atm_design...als/plate.html
http://www.minerals.sk.ca/atm_design/materials/bk7.html
Substrates page
An interesting thing is this sentence at the top of the page:
"As a telescope mirror material, BK7 is still much more expensive and inferior to many other possible choices."
I know this may make me look argumentative, but I like to get to the bottom of things when involved in discussions of a quantitative nature. I don't mean to disrespect you or anyone else.