XRT Technical Description

The XRT is a focusing X-ray telescope with a 120 cm$^2$ effective area, 23.6 arcminute FoV, 15 arcsecond resolution (half-power diameter, HPD) at 1.5 keV, and a 0.2-10 keV energy range. The XRT uses a grazing incidence Wolter 1 telescope to focus X-rays onto a 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 are the FM3 units built, qualified and calibrated as flight spares for the JET-X instrument on the Spectrum-X-Gamma mission. To prevent on-orbit degradation of the mirror module's performance, it is maintained at $293 \pm 5$ K with gradients of less than 1 K by an actively controlled thermal baffle. A composite telescope tube holds the focal plane camera, 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 to achieve a useful bandpass of 0.2-10 keV. The CCD consists of an image area with $600 \times 602$ pixels ($40 \times 40$ $\mu$m) and a storage region of $600 \times 602$ pixels ($39 \times 12$ $\mu$m). At launch the FWHM energy resolution varied from 50 eV at 0.1 keV to 190 eV at 10 keV. Below 0.5 keV the effects of charge loss to surface states in the silicon become significant. A special ``open-gate'' electrode structure gives the CCD-22 excellent low energy quantum efficiency (QE) while high resistivity silicon provides a depletion depth of 30-35 $\mu$m to give good QE at high energies. The detectors was intended to operate at approximately 173 K to ensure low dark current and to reduce the CCD's sensitivity to irradiation by protons (which can create electron traps which ultimately affect the detector's spectroscopy). During activation of the XRT the power supply for the thermoelectric cooler failed. XRT is currently passively cooled. Spacecraft pointings are monitored and in some cases restricted in order to keep XRT cooler than approximately 223 K.