We have embarked on a survey of ROSAT PSPC archival data with the aim of detecting all significant surface brightness enhancements due to sources in the innermost 15 arcmin of the PSPC field of view (0.5-2.0 keV). This project is part of the Wide Angle ROSAT Pointed Survey (WARPS) and is designed primarily to measure the low luminosity, high redshift, X-ray luminosity function of galaxy clusters and groups. The approach we have chosen for source detection [Voronoi Tessellation and Percolation (VTP)] represents a significant advance over conventional methods and is particularly suited for the detection and accurate quantification of extended and/or low surface brightness emission. In an extensive optical follow-up programme we are identifying galaxies, groups and clusters at redshifts ranging from z~0.1 to z~0.7. We present first results for an initial 17.2 deg^2 at detected fluxes > 3.5 x 10^-14 erg s^-1 cm^-2. We find the sky density of extended objects to be in the range 2.8 to 4.0 (+- 0.4) deg^-2. A comparison with a point source detection algorithm demonstrates that our VTP approach typically finds 1-2 more objects deg^-2 to this detected flux limit. The surface brightness limit of the WARPS cluster survey is ~1x10^-15 erg sec^-1 cm^-2 arcmin^-2, approximately 6 times lower than the EMSS. The WARPS LogN-LogS shows a significant excess over previous measurements for S > 8 x 10^-14 erg sec^-1 cm^-2.
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