Low-Thermal Expansion Material for Telescope Mirror Substrate Application

摘要

The material property and processability of the low thermal expansion glass-ceramics product by Ohara Inc. called CLEARCERAM~(~R)-Z were studied for telescope mirror substrate application. For material property, numbers of the key properties for the application, such as Coefficient of Thermal Expansion (CTE) characteristic in wide temperature range, Stress Birefringence and Mechanical strengths were intensively investigated focusing on the blank uniformity. The mean CTE of +0.15x10~(-7)/degree C in wide temperature range (-50 to +150degree C) with the standard deviation (Std.) of 0.03x10~(-7)/degree C and Young's Modulus & Poisson Ratio data with the coefficient of variation less than 1% were obtained for the blanks with the size up to Dia.670mm. The maximum Stress Birefringence was 3nm/cm within a 400mm square blank. For processability, the surface finish data of AFM Rms 0.15nm, the Power Spectral Density profile in the same level of low expansion amorphous glass and three dimensional structured samples were demonstrated. The comparison of the obtained data with known blanks specification for past and future telescope projects revealed that CLEARCERAM~(~R)-Z has capability to meet material property requirements for telescope mirror substrate application in the size up to Dia.670mm. Also for the precision metrology to support the material technology, CTE measurement system developed at OHARA was described.