Annu. Rev. Astron. Astrophys. 2000. 38: 289-335 Copyright © 2000 by Annual Reviews. All rights reserved

2.2. ASCA

ASCA, a joint Japanese-United States effort, was launched in early 1993. ASCA consists of four identical grazing-incident X-ray telescopes each equipped with an imaging spectrometer (Tanaka et al 1994). The focal plane detectors are two CCD cameras (known as the Solid-State Imaging Spectrometers, or SIS; Gendreau 1995) and two gas scintillation imaging proportional counters (Gas Imaging Spectrometer, or GIS; Ohashi et al 1996). The SIS detectors have superior energy resolution, whereas the GIS detectors provide a larger field of view. The angular resolution of ASCA is considerably worse than that of ROSAT, with a half power diameter of approximately 3'. However, ASCA's spectral resolution is much higher than that of the ROSAT PSPC (E / E ~ 20 for the SIS at 1.5 keV versus E / E ~ 3 for the PSPC), so this instrument has primarily played a role in the study of the spectral properties of the intragroup gas. Although the detectors aboard ASCA have undergone serious degradation, this mission is expected to remain operational until sometime in the year 2000.