The scientific payload of ROSAT consists of a 83 cm X-ray telescope (6 – 100 Å) and a 60 cm XUV-telescope (60 – 300 Å) which are looking parallel. An important objective of the mission is to perform the first all-sky survey with imaging X-ray telescopes providing an improvement in sensitivity by several orders of magnitude compared with previous surveys. A large number of new X-ray sources (~ 105) is expected to be discovered and located with an accuracy of 1 arcmin or better, depending on source strength. The sources discovered will represent almost all types of astronomical objects, ranging from nearby normal stars to distant quasars.
After completion of the sky survey which will take half a year, the instruments will be used for detailed investigations of selected sources with respect to spatial structures, spectra and time variability. In this pointing mode, which will be open for guest observers, ROSAT is expected to provide substantial improvements over the imaging instruments of the Einstein observatory.