A lightweight mirror made of a magnesium alloy for space-borne telescopes and instruments

We present a lightweight mirror made of a magnesium alloy for applications to space-borne telescopes and optics in instruments. A non-flammable magnesium alloy was recently developed. We consider this alloy to be a promising material for a lightweight mirror due to its high stiffness, high fracture toughness, low density, and suitability for machining. First, small flat mirrors were fabricated to obtain and improve the fabrication conditions. Then, a 175-mm magnesium-alloy mirror, with a spherical mirror surface and a backside hexagonal cutout structure to reduce the mass of the mirror, was designed and fabricated. The spherical mirror surface was fabricated using a diamond-turning process. The evaluated residual of the surface figure of the 175-mm mirror from the design is 0.90 μm rms. The deformation caused by the jig used for the fabrication was one of the major factors that increased the residual of the surface figure. The surface roughness of the 175-mm mirror was evaluated, and values of Sa = 9.6 nm and Sq = 12.6 nm were obtained by averaging the data of four measurements with a 167 μm × 167 μm field of view. Methods to improve the mirror surface are discussed. We also discuss space-borne telescopes and instruments for which this magnesium alloy is one of promising materials.