<title>Thermal analysis on hex-placement patterns of the Gemini primary mirrors</title>

Abstract

Ultra Low Expansion (ULE) material ofCorning Glass Work was chosen for the Gemini primary mirrors. The ULE mirror blank becomes monolithic by a fusion process which seals together 55 piece parts from a total of44 hexagonal segments (hexes). As a consequence ofthis fusion process, an optical surface distortion due to inhomogeniety in the coefficient of thermal expansion (CTh) is induced. The precise location ofthe individual hexes in the blank was determined by a detailed analysis in the optimization process. This analysis accommodates two thermal environments, thermal soak of -25'C and thermal gradient of3'C from the top to bottom surfaces. A parametric design study was conducted to determine an optimized pattern ofthe hex placements for the Gemini primary mirrors. Active optics corrections were performed to determine the optimum hex patterns. The results indicated that the optical surface distortion due to the CTE deviations was minimized based on the optimized location ofthe individual hexes. The thermal surface distortion and the optical image quality as well as the plate scale error ofthe primary mirrors met the design and the scientific requirements. The effect ofrandom errors ofthe CTE measurement was within the tolerance error budget.