Fabrication of ultrahigh precision master mandrel toward sub-10 arcsec x-ray Wolter mirror for x-ray telescope

X-ray telescopes with sub-10-arcsec angular resolution are required for detailed observations of high temperature celestial objects and high-energy phenomena in space. The angular resolution of an x-ray telescope depends on the figure accuracy of focusing optics, such as Wolter mirrors. In this research, a master mandrel was corrected to improve the figure accuracy of a Wolter mirror fabricated from the mandrel by nickel electroforming process. A nozzle-type elastic emission machining system was developed and applied to the figure correction over a limited area of the mandrel (110 mm in the longitudinal direction, 10° in the circumferential direction). The longitudinal figure accuracy of the mandrel was improved from 15–20 nm to ∼3 nm in Root Mean Square (RMS) value. A surface profiler system was developed to evaluate the longitudinal figure accuracy of Wolter mirrors. The measurement repeatability was 0.8 nm (RMS), which is sufficient for the evaluation of the figure accuracy. The figure accuracy of the Wolter mirror in the corrected area was evaluated to be 9.8 nm (RMS), and the slope error was 6.2 μrad (RMS). The imaging performance of a 2° azimuthal range in the corrected area of the Wolter mirror was evaluated by an x-ray focusing experiment at the 1-km beamline of BL29XU at SPring-8. The angular resolution was 4.3 ± 0.3 arcsec in half power diameter, which agreed with the angular resolution evaluated by ray-tracing simulation with the figure profile of the Wolter mirror evaluated by the developed surface profiler system. This agreement demonstrates that the developed surface profiler accurately measures the figure errors that affect the optical performance of the Wolter mirror.