Pyrex or Not Pyrex
I had always felt that it was better to opt for a telescope mirror made of Pyrex rather than one made of soda-lime plate glass. The reason was the dimensional stability of Pyrex. But that may no longer be true. Something not commonly known is that about ten years ago, all Pyrex production in the U.S. was terminated due to E.P.A. concerns, and its production shifted offshore, much of it to China which has virtually no environmental laws. As a result, the quality of Pyrex blanks is now much poorer. In fact, Peter Ceravolo, a highly regarded telescope manufacturer, reported that he had one Pyrex mirror that was so inhomogeneous that the aluminized coating physically tore due to differential cooling. You may have noticed that a number of telescope manufacturers are now abandoning Pyrex as a mirror material. Unless the quality of the current Pyrex improves, it appears there is no major advantage over plate glass. The only option? Big bucks for a ceramic (Cervit) mirror or a Pyrex mirror from an optics manufacturer who performs careful quality analysis on the mirror blanks.