Quote:
Originally Posted by ballaratdragons
In Australia there is no real need for Pyrex. While it may be nice to have a Pyrex Primary it doesn't really matter. We don't go from 24 degrees inside the house to -20 outside.
Also, Corning Incorporated has stopped production of Pyrex and Flourite Crystals. They have moved into new materials. Once all the Pyrex and Flourite is gone, it's gone.
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Whether you can `get away ' with plate glass instead of Pyrex depends totally on the thickness of the glass. Plate glass bends three times the amount of Pyrex for a given Delta and with Pyrex you need to get your delta to ambient with in 2 degrees to get get the figure of the mirror as the maker intended. Anyone who has set up digital thermometers will know how hard it is to keep a mirror within 2 degree of ambient when the temperature is falling.
In the case of plate glass , the owner has very little chance of maintaining the intended figure if the temperature is falling fast enough. I happen to have a 38mm Plate 14" mirror in my scope at the moment and it gives me plenty of trouble at certain times of the year. Smaller thinner mirrors in the 8 to 10" range aren't so troubled becasue of their fairly low mass. It also depends very much on how discriminating the observer is of course.
The idea that the world is running out of Pyrex is a bit of a USA forum beat up. Supplies of 2" sheet Pyrex are indeed over. Corning have a new plant in China and Schott are ramping up their production. If the industry want to cling to 2" pyrex blanks their going to have to get them moulded. Theres more Pyrex thanever in production now.
