Optical observations of a ROSAT sample of clusters of galaxies. I. 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 construct large unbiased samples. We present here the first results from optical follow-up observations of a ROSAT subsample of 42 clusters of galaxies in a contiguous area covering about 1700 square degrees. The general detection and selection procedures from the ROSAT All-Sky Survey adopted for clusters are described in detail. Further optical identifications of the cluster candidates and the measurement of their redshifts at the 3.6m ESO Telescope are presented. Various optical/X-ray correlations are investigated. Special care is given to the study of the significance of the observed cluster X-ray morphologies. X-ray contour plots for all clusters are shown, and for 31 objects, overlays on top of CCD images are presented in order to outline the correlation between optical and X-ray morphologies in detail and to flag possible contamination by unrelated point-like objects.