Optical glasses and glass ceramics for large lenses and prisms

Schott had delivered blanks for large lenses and prisms since many decades. Glass and glass ceramics objects with dimensions above 300 mm diameter or edge lengths will remain challenges for a glass manufacturer. This holds especially when the quality specifications exceed the standard level significantly. The developments in glass manufacturing allow casting of ZERODURR glass ceramic blanks up to about 1.5 m with homogeneities like that of high grade optical glass. This was utilized to test a convex mirror using a large Zerodur element in transmission providing the concave interferometer reference surface and the imaging aspherical lens simultaneously. Optical glass blocks of more than half a ton have been produced with outstanding internal quality. Although the manufacturing process is well controlled there are restrictions on the availability of such objects (glass types, long process times e.g.). The glass production process is presented pointing out its implications as a guideline for designers in order to avoid unneccessary time losses.