Spin-cast ZERODUR mirror substrates of the 8 m class and lightweight substrates for secondary mirrors

Besides many other applications, ZERODUR is an excellent material for mirror substrates of astronomical telescopes. The new requirements in the present day development of modern astronomical telescopes are discussed, including the needs for larger primary mirrors (8-meter class) and lower weight secondary mirrors. Compared to conventional ones, new manufacturing techniques are considered for the manufacture of thin ZERODUR meniscus- shaped mirror substrates in the 8-meter class and for the production of lightweight secondary mirror substrates. The different steps described for the ZERODUR lightweight substrates include forming of the lightweighted structure during casting, fusion of individual components to a total structure, and lightweighting of a massive block by various mechanical machining methods. (C.E.)