OPTICAL OBSERVATIONS OF A ROSAT SAMPLE OF CLUSTERS OF GALAXIES .1. TECHNIQUES AND X-RAY OPTICAL ANALYSIS

The applications of clusters of galaxies to cosmology are multiple and currently of special importance; it is thus of great interest to cons truct large unbiased samples. We present here the first results from o ptical follow-up observations of a ROSAT subsample of 42 clusters of g alaxies in a contiguous area covering about 1700 square degrees. The g eneral detection and selection procedures from the ROSAT All-Sky Surve y adopted for clusters are described in detail. Further optical identi fications of the cluster candidates and the measurement of their redsh ifts at the 3.6m ESO Telescope are presented. Various optical/X-ray co rrelations are investigated. Special care is given to the study of the significance of the observed cluster X-ray morphologies. X-ray contou r plots for all clusters are shown, and for 31 objects, overlays on to p of CCD images are presented in order to outline the correlation betw een optical and X-ray morphologies in detail and to flag possible cont amination by unrelated point-like objects.