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How did Pyrex glass improve the construction of large telescope mirrors?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

Pyrex, a type of borosilicate glass, significantly improved the construction of large telescope mirrors due to its low coefficient of thermal expansion.12

Unlike common silicate glass, which expands and contracts substantially with temperature fluctuations, Pyrex expands only about one-third as much when heated. This property is critical for large astronomical mirrors because it minimizes dimensional changes caused by temperature shifts, which can otherwise distort the mirror's surface and degrade image quality.314

By providing superior resistance to thermal shock and maintaining greater structural stability, Pyrex enabled the creation of massive, high-precision optics. This technology was a major advancement for 20th-century astronomy, most notably utilized in the 5-metre (200-inch) primary mirror of the Hale Telescope at the Palomar Observatory, as well as the 6-metre reflector at the Special Astrophysical Observatory in Russia.3412