Thermal Analysis on Hex Placement Patterns of the Gemini Primary Mirrors

Ultra Low Expansion (ULE) material of Corning 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 of 44 hexagonal segments (hexes). As a consequence of this fusion process, an optical surface distortion due to inhomogeniety in the coefficient of thermal expansion (CTE) is induced. The precise location of the 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 of 3’C from the top to bottom surfaces. A parametric design study was conducted to determine an optimized pattern of the 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 of the individual hexes. The thermal surface distortion and the optical image quality as well as the plate scale error of the primary mirrors met the design and the scientific requirements. The effect of random errors of the CTE measurement was within the tolerance error budget.