1. The X-Ray Telescope

The X-Ray Telescope (XRT) consists of four identical X-ray telescopes with focal length 3.5 m. The telescopes are mounted on an extendable optical bench which was expanded after launch to place the focal plane at the detectors. Each telescope is made up of 120 concentric aluminum foils 125 $\mu$m thick which are nested about 1 mm apart. Like those in the BBXRT mirrors, the foils are arranged in a conical approximation to the Wolter geometry. X-Rays are reflected at grazing angles of 0.5^$\prime$ by a 60 nm layer of gold. The pass band is 0.5-12 keV and the spatial resolution is 2.9^$\prime$ (half-power diameter). The telescope's plate scale is such that this corresponds to 2.9 mm at the focal plane.