X-ray observations of objects at cosmological distances from the 'Einstein' Observatory

Results of X-ray observations made by the orbiting Einstein Observatory of extragalactic objects, many of them at cosmological distances, are summarized. Following a brief review of the instruments on board the satellite, which is based on focusing X-ray optics, consideration is given to X-ray investigations of clusters of galaxies, the detection and study of X-ray emission from known and newly discovered quasars, and a deep survey to determine the contribution of discrete sources to the extragalactic X-ray background. It is noted that the Einstein Observatory has provided an observational capacity similar to those avaliable in the optical and radio ranges which is valuable in the study of high-energy processes and objects at cosmological distances.