Wolter mirrors fabricated by high-precision Ni electroforming process have been applied as focusing optics for x-ray telescopes. The typical replication accuracy is on the order of 100nm. For higher resolution observations, the figure accuracy is required to be improved. Recently, we have been developing an efficient figure correction method using an Si layer on Wolter mirror. Film thickness of Si can be measured with accuracy of 1nm level by thickness measurement gauge. Si is removed under wet process so that the figure accuracy improves. In this study, we developed a fluid jet polishing system especially for removing Si layer on the inner surface of Wolter mirrors. Surface roughness remained unchanged at 0.3nm in RMS (root mean square) value before and after processing to a depth of 133nm. For demonstration, a sine curve with a length of 10mm and PV (peak to valley) of 160nm was processed on Si on a plane surface, resulting in a processing accuracy of 25nm in PV and 6.7nm in RMS.
Ellipsoidal mirrors are promising focusing devices for soft x-rays. Ellipsoidal mirrors were fabricated by our group using nickel electroforming, for which a precise master mandrel is required because the surface accuracy of the reflective surface is limited by the mandrel. The aim of this study was to develop a processing method for correcting the surface figure error of the mandrel with high spatial resolution. This method is based on loose abrasive machining with organic abrasive slurry and a small rotating tool. We call this method “Organic Abrasive Machining” (OAM). The organic particle materials are acrylic resin or urethane resin, which have excellent characteristics that include such as high dispersion, low hardness, and high washability. In the presentation, we will report the basic performance of the newly developed OAM system, including such as a stationary spot profile, surface roughness, and long-term stability of the removal rate during figure corrections. Spatial wavelength of 100 m on a rod lens was achieved, which means that the OAM could be applied for deterministic figure correction of the surface waviness of ellipsoidal mirrors.
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