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 a CFRP substrate is a few µm, we have to make the smooth surface for reflecting X-rays on the CFRP substrate. We have developed a new method of shaping the reflective surface instead of the replica method used in lightweight X-ray mirrors such as Astro-H. In the new method, the reflective surface is formed by pasting thin sheet-glasses with 100 µm thick onto the CFRP substrate. The thin sheet-glass has a surface roughness about 0.4 nm as measured by Zygo. We fabricated a CFRP mirror pasting thin sheet-glasses, and then coated tungsten on the mirror in June 2020. The figure error (s) of the CFRP mirror was achieved to be about 1-2 μm by stacking the CFRP mirror on the housing module. X-ray imaging quality of the CFRP mirror was measured at Spring-8 in July 2020. The half-power diameter of the CFRP mirror was estimated to be about 150 arcsec, which was nearly equal to the prediction from a distribution of the slope error deduced from the surface profile. We describe a future plan to improve the image quality of the CFRP mirror.
Carbon-fiber-reinforced plastic (CFRP) has a higher strength-to-weight ratio and forming flexibility than metals, making it suitable for fabricating lightweight x-ray mirrors. However, CFRP has the disadvantages of print-through and deformation due to moisture absorption, which have prevented its use in optical mirrors. To expand the application of CFRP, we studied the formation of a moisture barrier layer on CFRP substrates. We formed a flattening layer a few micrometers thick on a CFRP substrate, following which we coated the substrate with SiOx as a moisture barrier. The effect of moisture absorption was then evaluated using accelerated aging tests. We found that the diffusivity of the CFRP substrate at 60°C and a relative humidity of 100% was ∼2 × 10 − 6 mm2 h − 1, which is 1/500th that of the barrier-less substrate. In the tests, the moisture absorption rate increased after ∼800 h. As we observed cracks on the flattening layer after 600 h, the rate increase could be associated with these cracks. Considering the damage to the barrier layer, we propose a modified model for the time profile, which is congruent with the observed time profile of the moisture content.
We fabricated X-ray mirrors from carbon-fiber-reinforced-plastic (CFRP) with a tightly nested design for X-ray satellites. The mirror shape is Wolter type-I quadrant shell geometry with a diameter of 200 mm and a focal length of 12 m. The mirror substrates were successfully formed with a rms error of about 1 μm. Through a replication process, a smooth surface was obtained on the CFRP substrate. We are developing a positioning method of thin mirrors in a telescope housing. It is found that a piezo-linear motor is very useful to adjust the mirror position with accuracy of sub μm. The CFRP mirrors were evaluated by using 20 keV X-ray pencil beam at BL20B2 in SPring-8 synchrotron radiation facility. The HPD of the mirrors was estimated to be about 2.3 arc-minutes. The spread of X-ray image would be caused by small waviness on the mirror surface after replication.