Thermal Analysis on Hex Placement Patterns of the Gemini
Primary Mirrors
Myung Cho Gemini 8m Telescopes Project, 950 N. Cherry Ave., Tucson AZ 85719
Abstract.
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.
Appears in Proc. SPIE 2857, "Advanced Materials for Optics and Precision Structures".
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.