The ROSAT mission is governed by two scientific objectivies, the first all-sky survey with an imaging X-ray and EUV telescope and the detailed study of selected X-ray und EUV sources.
As the primary objective ROSAT has performed in a scan mode the first all-sky survey with imaging telescopes in the soft X-ray band of 0.1 keV - 2 keV (corresponding to wavelengths of 100 Å - 6 Å) as well as in the adjacent extreme ultraviolett region of 0.04 keV - 0.2 keV (corresponding to wavelengths of 300 Å - 60 Å). This part of the mission lasted half a year, and was completed in February 1991. Thereby, sources have been recorded whose intensity is a hundred times weaker than the weakest sources in earlier X-ray surveys. The scientific harvest has been accordingly rich. More than 60,000 X-ray sources have been detected with the ROSAT all-sky survey , larger by almost two orders of magnitude than the 840 sources of the catalog of the previously largest all-sky survey of the HEAO-I satellite.
Following the all-sky survey, for over seven years to the present time, ROSAT continues to provide detailed observation of selected sources with respect to spatial structure, spectra and time variability. In this pointing mode the sensitivity has been at least two times larger than that of former missions. The location of the sources can be determined with an accuracy of at least 10 arcsec. The observation time is advertised and distributed world-wide to almost a thousand guest observers. All together, so far more than 9,000 pointed observations have been performed.