Abstract

Schott Zerodur and Corning 9600 are newly developed glass ceramics designed for high stability structures such as large mirror substrates. In early production these materials suffered from failure to return to length on thermal cycling, which can result in undesirable length and surface figure changes.¹ Recently, these materials have been greatly improved by their manufacturers. We present thermal expansion measurements of three vintages of both materials as they were cycled, at least twice, over the 100-475 K temperature range. Thermal equilibrium was obtained after every 25 K temperature increment. The newest Zerodur, called Zerodur M, is virtually free from hysteresis. However, to make this possible the general shape of its cte vs temperature curve has been greatly changed from that of standard Zerodur. (This leads us to wonder whether its homogeneity has suffered). The 9600 material still exhibits a few ppm of hysteresis, but compared with Zerodur M, 9600 maintains its low expansivity to much lower temperatures (100 K).

© 1989 Optical Society of America