The XRT is a focusing X-ray telescope with a 110 cm2 effective area, 23.6 x 23.6 arcmin FOV, 18 arcsec resolution (half-power diameter), and 0.2-10 keV energy range. The XRT uses a grazing incidence Wolter 1 telescope to focus X-rays onto a state-of-the-art CCD. The complete mirror module for the XRT consists of the X-ray mirrors, thermal baffle, a mirror collar, and an electron deflector. The X-ray mirrors (left) are the FM3 units built, qualified and calibrated as flight spares for the JET-X instrument on the Spectrum X-Gamma mission (Citterio et al. 1996; Wells et al. 1992; Wells et al. 1997). To prevent on-orbit degradation of the mirror module's performance, it is be maintained at 20 +/- 5 degrees C, with gradients of <1 degree C by an actively controlled thermal baffle (purple, in schematic below) similar to the one used for JET-X. A composite telescope tube holds the focal plane camera (red), containing a single CCD-22 detector. The CCD-22 detector, designed for the EPIC MOS instruments on the XMM-Newton mission, is a three-phase frame-transfer device, using high resistivity silicon and an open-electrode structure (Holland et al. 1996) to achieve a useful bandpass of 0.2-10 keV (Short, Keay, & Turner 1998).

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