Large mirrors with lightweight structure, such as those used in the telescope system of astronomy or spaceborne applications, are susceptible to stress caused by fabrication process. Furthermore, both the residual stress and subsurface damage are critical for the lightweight tooling of glass materials.

In order to figure out the stress distribution on glass substrate, the photoelastic method has been applied to not only the astronomical optics but also the industrial optics of semiconductor equipment. However, there are several influence factors in photoelasticity measurement, including the structure of mirror, fabrication process, and metrology technique. The above factors may affect to the retardation result of photoelasticity measurement and cause the error of stress calculation. Therefore, it is important to clarify the retardation difference contribution to the corresponding influence factors.

In this study, we attempted to use photoelastic instrument to investigate the relationship between the photoelastic effect and stress of several kinds of lightweight mirrors. There are three different lightweight mirrors were adapted to the photoelasticity measurement, including: (1) GSO 12” Mirror with 12 inches diameter made by fused silica, (2) Primary Mirror (M1-B) of Formosat-5 with 450 mm diameter made by ZERODUR^® , (3) Primary Mirror (M1) of Mircrosat with 380 mm diameter made by CLEARCERAM^®-Z. The experimental results depict some obvious retardation differences caused by the certain influence factors and the details will be discussed below.

Date Published: 14 September 2018
PDF: 6 pages
Proc. SPIE 10742, Optical Manufacturing and Testing XII, 107421N (14 September 2018); doi: 10.1117/12.2326972

Published in SPIE Proceedings Vol. 10742:
Optical Manufacturing and Testing XII
Ray Williamson; Dae Wook Kim; Rolf Rascher, Editor(s)