The
XRT
is a focusing X-ray telescope with a 120 cm 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
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
pixels (
m) and a storage
region of
pixels (
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
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.