# Zerodur

Zerodur (notation of the manufacturer: ZERODUR®), a registered trademark of Schott AG,[1] is a lithium aluminosilicate glass-ceramic[2] produced by Schott AG since 1968.[3] It has been used for a number of very large telescope mirrors including Keck I, Keck II,[4] and SOFIA. 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.[5] 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 $\mathrm{CTE}\pm 0.007\times10^{-7} /K$ allows for highly accurate applications which require high-precision.

Zerodur has both an amorphous (vitreous) component and a crystalline component. Its most important properties[6] are:

## Applications

The Keck II Telescope showing the segmented primary mirror made of Zerodur
1. Optics
2. Microlithography
3. Measurement technology[3]