Current status of development of lightweight x-ray mirror with carbon fiber reinforced plastic (CFRP)

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 similar to 100 mu m 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 mu m 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.