An assessment of the completeness and correctness of the Abell catalogue

Models of the galaxy distribution are used to investigate the effects caused by the projection of non-cluster members on galaxy clusters. The models give estimates of the completeness of the Abell catalogue and its correctness in the selection of real clusters. We estimate that the Abell catalogue contains 70 to 85 per cent of the rich clusters within z = 0.19 and also a significant fraction of false (i.e. non-physical) clusters. In the models 15 to 25 per cent of rich clusters have a true galaxy membership that is less than half the observed membership. We present the hypothesis that the excess of blue galaxies discovered in distant clusters by Butcher & Oemler is caused by miscalculation of the cluster membership.