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We present the measurements of the spot size of a powerful X-ray generator at an ISAS 30m X-ray beamline facility. The generator was manufactured by Rigaku corporation. A parallel light is ideal to measure the point spread function of an X-ray telescope. However, we cannot make purely parallel light because we have to use an X-ray source with a finite size at a finite distance in a ground facility. When we evaluate a performance of X-ray telescope on the ground, an apparent size of an X-ray source often limits measurements of its point-spread function (PSF). The smaller X-ray source at the larger distance is needed to measure the PSF core of X-ray telescope. A high power X-ray source is also need to illuminate a telescope with a raster scan method that allows us precise measurements of a PSF tail. Therefore a powerful X-ray generator with a small spot size is strongly favoured in terms of the measurements of X-ray telescopes. We then introduced a powerful 18 kW rotating anode generator with a fine point focus at ISAS in 2016. The minimum and maximum voltage is 5 and 60 keV, respectively. A small spot size is technically more difficult on a lower voltage. In order to measure the spot size of the generator, we place a pinhole with a 0.1 mm diameter at 12.7 m away from the generator. The flipped image was taken at a CCD imager place at 33.5 m away from the generator. We made measurements of the Cu-K line at 8 keV with two options. One is a high-power mode (normal filament option) with a wider spot size whereas the other is a low-power mode with a finer size (fine filament option). As a result, we found that a spot size of the normal and fine filament options are 2.8 mm and 0.80 mm (FWZI), respectively. We supplied a bias voltage of 800 V for the fine filament option. The spot size of 0.8 mm is equivalent to the angular extent of as good as 6 arcsecs in the ISAS X-ray beamline facility.
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