Editing Pyrex (section)

== Use in telescopes ==
[[File:Hale telescope mirror during grinding 1945.jpg|thumb|alt=A black and white photograph of a large, flat mirror with a geometric pattern, supported on a metal frame with several people surrounding it.|Pyrex glass used on the mirror of the [[Hale Telescope]]]]

Because of its low expansion characteristics, borosilicate glass is often the material of choice for reflective optics in astronomy applications.

In 1932, [[George Ellery Hale]] approached [[Corning Incorporated|Corning]] with the challenge of fabricating the {{convert|200|in|m|adj=on}} telescope mirror for the [[California Institute of Technology]]'s [[Palomar Observatory]] project.<ref name=glassgiant>{{cite web|title=The Glass Giant|url=http://www.cmog.org/article/glass-giant|website=Corning Museum of Glass|access-date=30 January 2015}}</ref> A previous effort to fabricate the optic from [[fused quartz]] had failed, with the cast blank having voids. The mirror was cast by Corning during 1934–1936 out of borosilicate glass.<ref name = Caltech>{{cite web|title=A History of Palomar Observatory|url=http://www.astro.caltech.edu/palomar/about/history.html|website=Palomar Observatory|publisher=California Institute of Technology|access-date=5 June 2015|date=28 May 2015}}</ref> After a year of cooling, during which it was almost lost to a flood, the blank was completed in 1935. The first blank now resides in the [[Corning Museum of Glass]].<ref>{{cite web|title=200-inch Disk|url=http://www.cmog.org/artwork/200-inch-disk |publisher=Corning Museum of Glass|access-date=30 January 2015}}</ref>