Error budgets for the image degradation of x-ray telescope on board Astro-E2

Abstract

X-ray telescopes (XRTs) of nested thin foil mirrors were developed for Astro-E, the fifth Japanese x-ray astronomy satellite. Although the launch was not successful, the re-flight of Astro-E mission is approved as Astro-E2 and will carry the same XRTs. Ground-based calibration of Astro-E XRT revealed that its image quality and effective area are somewhat worse than what are expected from the original design. Conceivable causes of these defects of the XRT performance (i.e., surface roughness, waviness, misalignment of reflectors, and so on) are examined by X-rays and optical microscopic measurements. In this paper, we distinguish quantitatively these causes to limit the performance of the Astro-E XRT. Using the detail measurements, we can attribute both degradation of the image quality and a deficit of the effective area from the design values mainly to a slope error with a mm scale in each reflector and shadowing effects of neighboring reflectors due to various factors. There is still room for improvement in the support system of reflectors (i.e., alignment bars) in the XRT. One of the main aims of the mirror system calibration is to construct response function. Therefore, it is important that the development of a representative numerical model and its validation against extensive ground-based calibration. Taking account of the results of the pre-flight calibration and the microscopic measurements, we develop and tune a ray-tracing simulator which constructs the XRT response function for a point source at an arbitrary off-axis angle and spatial distributions of celestial X-ray sources.