Abstract: Marshall Space Flight Center’s (MSFC) X-ray and Cryogenic Test Facility (XRCF) has tested the optothermal stability of two low-CTE, large-aperture mirrors in a thermal vacuum chamber. The mirrors deformed from several causes such as: thermal gradients, thermal soaks, coefficient of thermal expansion (CTE) gradients, CTE mismatch, and stiction. This paper focuses on how the aforementioned conditions affected the surface figure of the large optics while in vacuum at temperatures ranging from 23…