The X-ray telescope (XRT) [Aschenbach1988] consists of a fourfold nested Wolter 1 type mirror with an entrance diameter of 80 cm and 3 focal plane detectors. Two of these are position sensitive proportional counters (PSPC) [Pfeffermann et al.1986] with a positional resolution of about 20 arcsec and a moderate energy resolution giving 4 to 5 bands within the 0.1 to 2.4 keV energy range. This detector has a field of view of 2 degrees and has been used during the all sky survey (one of the two redundant PSPC detectors was lost on Jan 25, 1991; due to the exhaustion of the counter gas resources the last observations with the remaining PSPC were done in mid 1994). The third detector is a High Resolution Imager (HRI) [David et al.1993] channel plate detector allowing resolution of a few arcsec without energy information. It is a rebuild of the HRI on-board the EINSTEIN observatory.
The XUV telescope or Wide Field Camera (WFC) [Barstow and Willingale1988] is a Wolter 1 type telescope with 2 channel plate detectors in the focal plane. The positional resolution is about 1 arcmin in the center of the 5 degrees-wide field of view. Filters defining 4 wavelength bands are used to obtain spectral information in the range 20 to 200 eV (60 to 600 Å). The sensitivity of the WFC detector has degraded substantially in the event of January 25, 1991.