ROSAT was conceived for two tasks; during the first six months following its launch on 1 June 1990, it surveyed the full extent of the heavens with its Position Sensitive Proportional Counter (PSPC) detector equipment to complete the first ever all-sky survey using an imaging X-ray telescope. During this process, it detected and catalogued approximately 80,000 X-ray sources. Parallel to this, the UK Wide Field Camera recorded 6000 sources in the extreme ultraviolet regime of the electromagnetic spectrum.
The advantage of this all-sky survey was twofold: the large number of objects observed provided a sound basis for extended statistical investigations. Secondly, it provided a virtually unlimited field of view that enabled the observation of extended objects such as nearby supernovae remnants. The numerous X-ray sources localised in the all-sky survey established the basis for subsequent detailed observations of individual objects. Since many X-ray sources are variable over time much of this data is still used for further scientific investigations today.
During its second phase, which lasted until the end of 1998, detailed observations of individual X-ray sources were carried out (pointed observations). At this time, ROSAT was being operated as a public-domain observatory, available to every interested scientist on submission of a well-founded application. Under the auspices of the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching, near Munich, a guest observer programme was established, with a participation of about 4000 scientists from 26 countries.
During its eight years of service, ROSAT greatly outperformed the original expectations of the scientific community through the vast number of excellent scientific observations it provided. Almost all known kinds of astrophysical objects could be observed with the help of ROSAT, mainly those experiencing high-energy processes or with temperatures of several million degrees Celsius. Examples of this include active galactic nuclei that seemingly contain black holes, as well as galaxies and galaxy clusters. In our own Milky Way, the objects observed included normal stars, X-ray binaries, neutron stars and supernovae remnants. Not only that, but objects in our own Solar System such as the Moon, planets and comets, surprised observers by proving to be X-ray sources as well.
The results have been documented in more than 8000 publications. Observations at other wavelengths (visible, radio, infrared and gamma radiation) supplement the information relating to these astronomical objects. The ROSAT catalogue of pointed PSPC observations, revised in 2001, includes the positions and count rates of more than 100,000 X-ray sources.