Forming mandrels for X-ray telescopes made of modified Zerodur

摘要

For the next generation of X-ray observatories (CONSTELLATION-X and XEUS) a mass production of glass mirror segments is considered. The mirror substrates (SCHOTT D263 and SCHOTT Borofloat~(~R) 33) will be pre-shaped in a high temperature slumping process by use of precision forming mandrels. SCHOTT GLAS developed the glass ceramic material Zerodur~(~R) K20 to meet the requirements of these mandrels. The new material is a modification of the well-known Zerodur~(~R). A heat driven transformation thereby changes the crystalline phase from high-quartz to keatite structure. The resulting Zerodur~(~R) K20 exhibits an increased stability at high temperatures of up to 850℃ and a low thermal expansion coefficient (CTE) of approximately 20·10~(-7)K~(-1) (20°-700℃). Numerical simulations of the slumping process based on experimental parameters of Zerodur K20 and the mirror substrate materials are presented.