Optical performance of aluminum mirror for cryogenic applications

Aluminum mirrors are most commonly used for cryogenic optic systems. A parameter simulation has been conducted to design the aluminum mirror. We have obtained an optimized mirror structure after optimization applied in the mirror design stage. A finite element model analysis was conducted to predict the gravity sag, mounting-induced error and cryo-deformation of the mirror. Then we have fabricated a 160 mm diameter aluminum mirror based on the optimization results, tested it at room and cryogenic temperature. An error budget developed based on the experimental results shows that the fastening preloads effect dominates the surface figure of the aluminum mirror. The total deformation of the mirror surface at 100 K meets the optical requirement, which indicates that the optimized aluminum mirror is stable enough to retain good surface figure at cryogenic temperature and the aluminum mirror is promising for its cryogenic applications.