The determination of cluster properties and their evolution provides fundamental insights into the formation of large scale structure in the universe.
Clusters are ideal for testing cosmological models.
Further, the masses and scales of clusters are such that they should comprise a fair sample of the mass of the Universe, with representative fractions of the different components. Since cluster formation by gravitational collapse is fairly well understood, comparisons of cluster properties with theoretical predictions for different cosmological models can be used to distinguish among these models. All these groups work by searching large numbers of PSPC fields using various algorithms to detect extended sources. Easy access to the data enables different algorithms to be tried and compared leading to more reliable results. This provides evidence independent of the SNIa and CMB results for either a low or a non-zero cosmological constant.