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Forty years of ZERODUR mirror substrates for astronomy: review and outlook

Initiated in 1968 by the first order of the Max-Planck-Institute in Heidelberg the successful history of ZERODUR® continues now since 40 years. ZERODUR® zero expansion glass ceramic from SCHOTT has been the material of choice in astronomy for decades, thanks to its special properties such as its extremely high thermal and mechanical stability. Today most of the major modern optical telescopes of the 4 m class and of the 8 m to 10 m class are equipped with ZERODUR®. For the future several Extremely Large Telescope (ELT) projects are in development, which are designed with even larger primary mirrors ranging from 30 m to 42 m. Also here ZERODUR® is under consideration. A historical review, the actual status of developments and an outlook to the future is given in this paper.