Hi Ian
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Originally Posted by IanW
I agree in general with what you say, however I'm not so sure about the cool down times as from memory Zerodur has a higher Thermal Conductivity level than Pyrex 740.
SNIP
I really don't have much knowledge about thin mirrors over 16" as I use full 1:6 ratio blanks for mirrors over 16" diameter and never pyrex which while it's a killer material for smaller mirrors does have sag issues with larger sizes.
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Zerodur is a little stiffer than Pyrex but nothing a slight increase in thickness of Borosilicate can't replicate .
Emmisivity and conductivity are not the same thing. Pyrex is about 33% more cunductive but Zerodur is worse in the emmisivity value, so although it will be slower to take up heat , it is worse at shedding it.
Most altaz scope use 10: 1 or less these days , particularly as it is easy to computer optimise a mirror cell. A 20" X 1.6" mirror has a PV error of about lambda/60 surface flexure on an 18 point cell, and will yield a textbook diffraction pattern under good seeing at all but near the horizon. Most of the old scopes with thick mirrors have serious thermal equalisation problems. Web-back or honeycomb blanks are more common in the Equatorial mounted observatory scopes, but they are very expensive. A 24" Hextek blank runs at $25 K USD still cheaper than a Zerodur blank.
Mark