Abstract
The influence of the heat transfer conditions on the back surface of a large parabolic mirror on the thermally induced shift in focus when the working surface absorbs a portion of the incident radiation was investigated. In addition, a comparison of the thermal aberration of the mirror under the conditions of natural heat transfer on the back and working surfaces when the back surface is thermally insulated or thermally stabilized was carried out. As a result of this research, it was shown that, for mirrors made of materials with low thermal conductivity, such as Zerodur or Sitall, the thermal insulation of the back surface leads to an approximate 40-fold decrease in the thermally induced focus increment, which is due to the compensation of the flexural deformation of the mirror. For a mirror made of a material with high thermal conductivity (such as silicon carbide), the thermal insulation of the back surface has the opposite effect. At the same time, the gain from the thermal stabilization of this surface is small—a factor of only 1.36. The results and conclusions presented herein are relevant for mirrors used in both ground- and space-based telescopes, operating under conditions when absorbed radiant flux density values do not exceed 50 W/m2. The proposed calculation method is convenient for determining ways to reduce the thermal aberration in mirrors.
© 2019 Optical Society of America
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