D. Vukobratovich and A. Gerzoff
National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson AZ 85719
Gemini 8m Telescopes Project, 950 N. Cherry Ave., Tucson AZ 85719
The optical surfaces of metal mirrors are often plated with electroless nickel to reduce light scattering. The thermal coefficient of expansion of electroless nickel, 13.5 x 10 m/m-K, is significantly different from that of a typical mirror substrate material such as aluminum. A change in temperature produces a "bi-metallic" bending stress in an electroless nickel plated mirror, which can induce optical surface distortion. Possible solutions to the "bi-metallic" bending effect include: metal matrix composites with a thermal coefficient matched to that of the plating, thick mirrors with sufficient stiffness to resist bending, symmetric cross sections producing equal and opposite bending, and plating of both sides of the mirror to balance bending deformations. These solutions are compared using a design example from a cryogenic instrument, the Gemini Near Infrared Spectrograph (GNIRS). Deflections are calculated using both finite element and closed form solutions. The closed form solution produces an order of magnitude estimate, which may not be a reliable guide to the actual therm-optic performance of a plated metal mirror. More sophisticated analytical techniques, which determine the actual type of optical surface error, such as focus, piston, and aberration terms, are required to determine the performance of a mirror undergoing "bi-metallic" bending.
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