Optical surveys for galaxy clusters

Optical surveys for galaxy clusters have been the major method for creating large cluster catalogs. We give a brief review of the history of optical searches of galaxy clusters, starting from that of Abell. The traditional application of this survey method suffers from contamination due to projection of galaxies along the line of sight, which becomes increasingly more severe at higher redshift. The new generation of wide-field CCD imagers has provided a renewed impetus for optical surveys for clusters. We describe a new cluster finding technique using the red sequence of early-type galaxies in galaxy groups and clusters, which eliminates the projection problem by essentially producing a 3-D distribution of red galaxies using two-filter imaging data. The Red-Sequence Cluster Survey (RCS) is a 100 square degree optical survey, carried out using 4m class telescopes, which is optimally designed to search for clusters at 0.5 < z < 1.4 utilizing this technique. We present preliminary results which indicate that the RCS is extremely efficient in detecting galaxy clusters at these redshifts, including a number of clusters with multiple strong lensing arcs.