ASTRO-E2 XRT precollimator for stray light protection I. Design and expected performance

Next Japanese ASTRO-E2 satellite carries five X-ray telescopes (XRTs), and the pre-collimators for the stray light protection will be installed on them. The pre-collimator is composed of the vertical foil cylinders which line up at the top of the primary reflectors of the XRT. This configuration is effective to reject the stray light of the 'secondary only' reflection, which is the most of the stray light from large off-axis angles up to 70' to the focal plane detector within 18' x 18'. If the height of the pre-collimator is 30 mm (15% of the total height of XRT), we can protect all the "secondary only" component at > 30' off-axis. The field of view will become only about 10% less due to the collimation effect of the 30 mm high pre-collimator. However, new stray light component is generated by the reflection and scattering by the pre-collimator itself, especially at small off-axis angles. As a result, we estimate the total flux of the stray light to be about 10% at 30' off-axis and 5% at 60' off-axis, compared with the case without pre-collimators.