Pre-Collimator of the Astro-E2 X-Ray Telescopes for Stray-Light Reduction

We present the design and fabrication of the pre-collimator for the X-ray telescope onboard the X-ray astronomy satellite Astro-E2, and its effect on stray-light reduction. The pre-collimator was designed to efficiently reduce secondary reflection, which is the brightest component of stray light, giving rise to a ghost image in the field of view of the focal-plane detector. With the pre-collimator, composed of blades with an effective height of 30 mm, we confirmed from X-ray measurements that the intensity of the secondary reflection is reduced by roughly a few orders of magnitude on average in the off-axis angle range of $20' \hbox{--} 70'$ . We also confirmed that the loss of the on-axis effective area due to installation of the pre-collimator is only $0.5 \pm 0.2\%$ . The field of view becomes narrower, but the reduction factor amounts only to 8%. Ray-tracing simulations of in-flight observations also demonstrate that introduction of the pre-collimator is expected to be highly significant for improving data quality, due to a reduction of secondary reflection.