The Rosat Observatory was launched on June 1st, 1990, and after a brief Performance Verification phase, performed an almost complete sky survey. One of the two main telescopes on the Observatory is the UK Wide Field Camera (WFC). The WFC is the first extreme ultraviolet (EUV) instrument to survey the whole sky, and during the survey detected several hundred active late-type stars.
The WFC (Wells et al., 1990) was built by a consortium of five UK institutes, four Universities and the Rutherford Appleton Laboratory. During the all-sky survey, two filters were used. These allow broad-band photometry to be performed. The SIA filter (lexan/carbon/boron) and S2A filter (lexan/beryllium) were alternated on a one day basis. The SIA filter covers the energy range 0.085-0.180 KeV, centred on 0.124 KeV, while the softer S2A filter covers the range 0.070-0.110 KeV (centred on 0.090 KeV) (10% efficiency) [see Kent et al. (1990) for a full description of the WFC filters].