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We present the current status of the pre-collimator for the stray-light reduction, mounted on the ASTRO-H
X-Ray Telescopes (XRT). Since the ASTRO-H XRTs adopt the conical approximation of the Wolter-I type
grazing incident optics, X-rays from a source located far from the telescope boresight create a ghost image in the
detector field of view (FOV) as a stray light, and then reduce the signal-to-noise ratio even in the hard X-ray
band. We thus plan to mount the pre-collimator, which is comprised of cylindrical blades aligned with each
primary mirror, onto the XRTs to remove the stray light. While the pre-collimator for the Soft X-ray Telescopes
is designed by the similar principle adopted for the Suzaku pre-collimator, that for the Hard X-ray Telescopes
requires some trade-off studies to select an appropriate blade material. The HXT pre-collimator currently utilizes
the aluminum blade with the 50 mm height and 150 μm thickness. We examined the observational effects by the
hard X-ray (> 10 keV) stray light and the expected performance of the pre-collimator in some scientific cases,
using a ray-tracing simulator. We found that the Galactic center may be mostly covered with the stray light
from the well-known bright X-ray sources. In addition, the flux estimation of the extended X-ray emission such
as the Cosmic X-ray Background is also found to have large (~ 30%) uncertainty due to the stray light from
the outside of the XRT FOV. The pre-collimator improves the situations; the stray light covering the source-free
region in the Galactic center can be reduced by half and the uncertainty of the flux determination for the diffuse
source decreases down to < 10%.
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