Fundamental study on optical performance of low-melting-point metal mirrors for space telescopes

The installation of telescopes with larger primary mirrors in outer space is required for the study of the early universe. However, the shipping, assembling and maintenance of the primary mirror are technical challenges. The primary mirror made of low-melting-point metal (LMPM) enables to simplify these procedures. The rotating liquid metal surface is shaped into a parabola and solidified on the site. The technological feasibility depends on the dynamic transformation of the parabolic LMPM mirror and its optical reflectance. The dynamic transformation of LMPM mirror was clarified by means of experiments with liquid gallium (Ga) which has a melting point of 302.91 K. The parabolic shape of solid Ga mirror was produced by the axial rotation and the solidification. The parabolic shape agreed with the theoretical curve obtained by a physical model. The reflectance spectra on the surface of various LMPMs solidified (i.e., Ga, lead (Pb), tin (Sn), bismuth (Bi), lead-bismuth eutectic (Pb-Bi) and Wood's metal (Bi-Pb-Sn-Cd)) were measured. The reflectance of Ga solidified was approximately 70 % and was not degraded in the near-infrared wavelength due to the favorable optical characteristics. The reflectance both for p-polarized and s-polarized light of Ga solidified was measured by the spectrophotometer with the polarizer at the angle of incidence of 30 degrees, 45 degrees, 60 degrees and 75 degrees. The trend of measured reflectance agreed with theoretical prediction.