Forming mandrels for x-ray telescopes made of modified Zerodur

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 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 K20 to meet the requirements of these mandrels. The new material is a modification of the well-known ZERODUR. A heat driven transformation thereby changes the crystalline phase from high-quartz to keatite structure. The resulting ZERODUR K20 exhibits an increased stability at high temperatures of up to 850°C and a low thermal expansion coefficient (CTE) of approximately 20•10 -7 K -1 (20°-700°C). Numerical simulations of the slumping process based on experimental parameters of Zerodur K20 and the mirror substrate materials are presented.