20 July 2000 Thermal control of classical astronomical primary mirrors
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We present the conceptual design of a thermal control system for the KPNO Mayall 4-m primary mirror. An electrical current is passed through the reflective coating to match the temperature of the front surface of the mirror to ambient. Cooling of the back with thermo-electric coolers provides a cold reservoir for when the front surface temperature must be decreased. Temperature response of the front is estimated from a 1D thermal study: a 1C increase can be produced in approximately 1 hour from 25 wm-2 of heating and a 1C drop in approximately 1 hour with a 10C temperature gradient front-back. Finite-element mechanical analysis is used to model optical affects and stress induced by a 10C temperature gradient through the mirror. Thermal print-through and mechanical stress are found to be negligible. The bending of the mirror by the front-to-back gradient is small and can be removed as a focus shift. In another study, modeling of heat transfer through free air convection simulates the response of a mirror without thermal control. For this case, the time to change the top surface temperature is long because of the mass of the mirror and the relatively poor transfer of energy by convection. For a hot mirror in a 5C cold environment to decrease the temperature by 1 C we estimate approximately 5 hours for the top temperature and approximately 24 hours for the core temperature. Image degradation from radial temperature distributions arising from convective heating or cooling of the mirror by surrounding air can become noticeable (0.05 - 0.1 arcsec FWHM, added in quadrature).
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Bruce Bohannan, Bruce Bohannan, Earl T. Pearson, Earl T. Pearson, David Hagelbarger, David Hagelbarger, } "Thermal control of classical astronomical primary mirrors", Proc. SPIE 4003, Optical Design, Materials, Fabrication, and Maintenance, (20 July 2000); doi: 10.1117/12.391531; https://doi.org/10.1117/12.391531

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