In the frame of an international R&D project, a team from Japan and Germany developed a replica process to manufacture a CFRP honeycomb mirror for visible application. The main target was to generate the optical surface with minimized effort to achieve a microroughness of better (1 – 2) nm RMS. This technology would enable the team to manufacture cost-effective mirrors for larger amounts, like for constellations. In this paper, we will present the recent results of this development and the finally achieved performance of a 300 mm light CFRP mirror. We will also present lessons learned for next steps of development to achieve such mirrors with an optical performance not only for microroughness but also for surface accuracy.
CFRP is a composite material composed of carbon fiber and resin. CFRP is commonly applied to the aerospace industry which requires lightweight and intensity. Thanks to superior formability of CFRP, we can form shape of Wolter-1 optics, which consists of paraboloid and hyperboloid, to a monolithic substrate. Since the surface roughness of CFRP substrate is a few microns, it is required to make the smooth surface for reflecting X-rays on the CFRP substrate. We have developed a new method of shaping the reflective surface by pasting thin sheet-glass with 50~100 μm thick onto the CFRP substrate. The surface roughness of the thin sheet-glass was measured to about 0.4 nm by Zygo. Our CFRP mirror is a candidate for backup mirrors in the FORCE mission, and are being developed for balloon-borne experiments planned in the near future. Current image quality of our CFRP mirror was measured to be about 60-120 arcsec by illuminating an X-ray pencil beam at the ISAS beam line. In order to achieve a high imaging quality less than 15 arcsec, we will improve the CFRP mirror surface using both the replica method, and an ultra-precision mold processed with a shape error of 0.1 µm or less. The mold will be completed in the summer of 2022. We report on the current status of the development of the CFRP mirrors.
The technology to manufacture main reflectors of satellite-mounted telescopes using CFRP (Carbon Fiber Reinforced Plastics) was studied. CFRP has high specific rigidity and zero thermal expansion which are required for space telescopes. However, the difficulty of high precision machining of CFRP and the fiber print-through appeared on the surface have prevented CFRP from achieving high accuracy and surface smoothness required for the mirrors. We studied the replica technology and process conditions in detail. The replica technology was improved not only to eliminate the fiber print-through on the CFRP surface, but also to achieve high shape accuracy of the mirrors. At the moment, shape accuracy of 0.6 μm and surface roughness of 3 nm were obtained. The area density of the CFRP mirrors was lighter than one-fifth of the conventional mirrors made of zero expansion glass which are machined for mass reduction. The lightweight and thermally stable CFRP mirrors are expected for reflectors of large aperture, and will improve the resolution of space telescopes.
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