On June 1 
, 1990 the ROSAT observatory
[Trümper1983]
was launched
successfully into a circular orbit of 575 km altitude and
53 degrees inclination.
The satellite carries two telescope systems on board: an X-ray telescope,
operating in the energy range 0.1 to 2.4 keV, and a smaller
XUV telescope sensitive in the range from about 20 to 200 eV.
ROSAT --an abbreviation for Röntgen Satellit--
is a German project with major
contributions from the United States and the United Kingdom.
Specifically the US (NASA/SAO) provided one of the three focal
detectors of the X-ray telescope and launched the satellite,
and the XUV telescope was built by a consortium of five British
institutes, namely the Universities of Leicester and
Birmingham, the Mullard Space Science Laboratory,
the Imperial College of Science Technology and Medicine and
the Rutherford Appleton Laboratory.
After completing an initial Calibration and Verification Phase (CV Phase)
ROSAT started on July 30, 1990, its main scientific task:
to perform the first all sky survey
with an imaging telescope both in the soft X-ray and the XUV regimes.
After this survey, which lasted six months,
an extended observatory program began (on a guest observer basis)
with pointings
to many individual targets.
In the meantime the 4 
pointed observation period (AO-4) has been
almost completed and a fith one (AO-5; HRI only) will begin
in October 1994.