Zerodur

Zerodur (notation of the manufacturer: ZERODUR®), a registered trademark of Schott AG, is a lithium aluminosilicate glass-ceramic produced by Schott AG since 1968. It has been used for a number of very large telescope mirrors including Keck I, Keck II, and SOFIA as well as some smaller telescopes (such as the GREGOR Solar Telescope). With its very low coefficient of thermal expansion it can be used to produce mirrors which retain acceptable figures in extremely cold environments such as deep space. Although it has advantages for applications requiring a coefficient of thermal expansion less than that of borosilicate glass it remains very expensive as compared to borosilicate. The tight tolerance on allows for highly accurate applications which require high-precision.