Exosat for ground-based observers.

Performance capabilities of the Exosat X ray satellite developed by ESA are described. Exosat will fly an elliptical orbit out to 200,000 km and in to 500 km, allowing 80 hr of observations beyond 50,000 km. On board are two Wolter Type I imaging telescopes with position sensitive detectors and channel multiplier arrays, a rotating bank of filters, and transmission gratings. Nominal resolution is designed for 9 arcsec FWHM over a 2 deg field, with the detectors operating in the energy range 0.04-4.5 keV. A medium range array of eight detectors is provided to view events in the 1.2-50 keV range. Finally, a gas scintillation proportional counter, operating from 2-40 keV, has a 10 percent resolution at 6 keV over a 160 sq cm area. It will be used to study line profiles of bright sources. The 80 hr observational period will enhance the chances that ground personnel will obtain valid X ray measurements.