Zerodur[2] is a lithium-
aluminosilicateglass-ceramic[3] produced by
Schott AG since 1968.[4] It has been used for a number of very large telescope mirrors including
GTC,
Keck I, Keck II,[5] and
SOFIA, as well as some smaller telescopes (such as the
GREGOR Solar Telescope). With its low
coefficient of thermal expansion (CTE), it is suitable for producing mirrors that maintain acceptable
figures in extremely
cold environments, such as deep space.[6] 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 CTE, ±0.007×10−6 K−1, allows for its use in high-precision applications.
Zerodur has both an
amorphous (vitreous) component and a
crystalline component. Its most important properties[7] are:
The material exhibits a particularly low thermal expansion, with a mean value of 0 ± 0.007×10−6 K−1 within the temperature range of 0 to 50 °C. This thermal expansion performance is comparatively superior to
that of fused quartz by two orders of magnitude.[8][9]
High 3D
homogeneity[9] with few inclusions, bubbles and internal stria (as contrasted to
Cer-Vit).