Zerodur

Zerodur has a near zero coefficient of thermal expansion (CTE), and is used for high-precision applications in telescope optics, microlithography machines and inertial navigation systems.

In space, it has been used for the imager in Meteosat Earth observation satellites,[15] and for the optical bench in the LISA Pathfinder mission.

Its most important properties[21] are: Schott began developing glass-ceramics in the 1960s lead by Jürgen Petzoldt, in response to demand for low expansion glass ceramics for telescopes.

[26] In 1966, Hans Elsässer, the founding director of the Max Planck Institute for Astronomy (MPIA), asked the company if it could produce large castings of almost 4 meters using low-expansion glass-ceramic for telescope mirror substrates.

The mirrors were delivered by late 1975,[26] and went into operation in 1984 in a telescope at the Calar Alto Observatory in Spain.

Opening of the ELT secondary mirror ZERODUR® blank mould containing the glass at first annealing at the Schott AG 4-meter blank annealing facility in Mainz, Germany. [ 1 ]
The Keck II Telescope showing the segmented primary mirror made of Zerodur