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Catalogues of galaxy clusters have been used to study large scale structure and evolution in the universe. There are, however, many problems with the traditional catalogues derived from a visual inspection of photographic plate material that can be significantly alleviated using X-ray selected samples. To solve some of these problems, an international collaboration has been formed to utilise the ROSAT X-ray all-sky survey data to create a catalogue of cluster candidates and determine their optical properties. The presence of extended X-ray emission is indicative of an intracluster medium and implies a bound system that is, or in the process of being virialised. This eliminates the problem of chance optical alignments which affect cluster richness measures and produce spurious clusters.
A preliminary catalogue using a flux limit of 2x10$^{-12}$ erg sec$^{-1}$cm$^{-2}$, a mininum diameter of 1 \arcmin, declination $>$ 0\deg $ $ and galactic latitude $>$ 20\deg $ $ has yielded 522 cluster candidates. A later version will relax these strict conditions and is expected to detect a few thousand lower luminosity and more distant clusters. Initial results have shown that this survey is detecting clusters as extended X-ray sources to redshifts of z $>$ 0.3 and is finding many previously unidentified clusters and groups of galaxies.
In this paper we present a status report on the optical studies of these clusters in which we are obtaining both redshifts and CCD images for the sample. These data can be used to determine luminosity functions for investigating evolutionary effects as a function of redshift and morphology, while their spatial and angular correlation functions are a measure of large scale structure.
