Design of the pre-collimator for the NeXT x-ray telescopes

We present the design of the pre-collimator for the X-ray telescopes (XRTs) onboard New X-ray Telescope (NeXT). The optical design adopted for the NeXT XRTs is conically-approximated Wolter-I type optics. The tightly-nested reflectors with thin substrates (150-300 μm) enable us to achieve the large effective area and extremely light weight simultaneously. However, due to the packed reflector shells, X-rays from the outside of the XRT field of view occasionally arrive at the focal plane without the normal double reflection (stray lights), and then produce a ghost image on the detector. Thus, the stray-lights contamination degrades sensitivity of the source detection In order to reduce the stray lights efficiently, we plan to mount a collimator onto each telescope (referred to as pre-collimator), which is similar to that equipped with the Suzaku XRTs. The pre-collimator consists of coaxially-nested cylindrical blades, each of which is aligned radially with the corresponding primary refector. We found the height of the pre-collimator blade to be ~100 mm that is required to block the X-rays with off-axis angles of 30'-50', which are the main ligth pass of the stray lights for the NeXT XRTs.