Imaging with insolated mirrors

Abstract

Modern solar telescope designs are different from the conven- tional concept of vacuum telescopes. These new designs are "open" telescopes which try to minimize the temperature difference between various parts of the telescope and the ambient air. In this paper, we address a few issues related to the thermal response and image quality of such insolated mirrors. We estimate the distortion produced by thermal and material inhomogeneities and present limiting values of allowable temperature differences and percentage change of expansion coefficients for different aperture diameters, for typical materials under best possible seeing conditions. We predict the evolution of surface temperature of an insolated mirror using a simplified theoretical approach and show that it is compatible with the experimental values to a large extent. The results indicate the possibility of avoiding active cooling of the mirror surfaces, at least for primary mirrors with aperture diameter less than or equal to 50 cm.