Abstract
Modern large telescopes require large mirrors for collecting light from faint stars and galaxies. The
mirrors must be machinable to form intricately curved smooth surfaces for telescope optics and must be
stable enough under the varying environment. Several types of zero thermal expansion glass are most
frequently used as the material for those mirrors. CLERCERAMTM-Z is one of such zero thermal
expansion glass ceramics manufactured by OHARA Inc. and adopted for the primary mirror segments
of TMT. The present article describes the development history, the manufacturing processes and various
characteristics of CLEARCERAMTM-Z. Measurement methods and actual measured data of thermal
expansion coefficient, processability, long term stability, and internal stress are presented to date,
200 blanks, out of 574 blanks needed, have been manufactured all meeting the specifications of TMT
primary segments.
