The X-Ray Telescope onboard Suzaku

Abstract We present the design parameters, production process, and in-flight performance of the X-ray telescope (XRT) onboard Suzaku. The imaging capability is significantly improved over the ASCA XRT, which had half-power diameters of ${3rlap {.}{}^{mathrm {prime }}6}$, to ${1rlap {.}{}^{mathrm {prime }}8}$–${2rlap {.}{}^{mathrm {prime }}3}$ for all four XRT-I modules. The optical axes are found to be distributed within a radius of ${1rlap {.}{}^{mathrm {prime }}3}$, which makes the observation efficiency of all the XRTs more than 97% at the XIS-default observing position. The vignetting over the XIS field of view predicted via ray-tracing coincides with that measured for observations of the Crab Nebula to within $sim 10%$. Contemporaneous fits of a power law to all of the XIS spectra of the Crab Nebula taken at the two standard observing positions (XIS/HXD-default positions) gives a flux consistent with that obtained by Toor and Seward (1974, AJ, 79, 995) to within $sim 2%$. The pre-collimator on the top of each XRT module successfully reduces the intensity of the stray light from the $20’$ and $50’$-off directions down to the level of pre-flight expectations.