Completeness of our survey defined as the fraction of all possible targets for which z was successfully estimated. Panel a: 2D distribution of the completeness on the sky for the MMT data set. The green circle has a radius equal to R₂₀₀ of the cluster (derived in Sect. 5.1). Panel b: completeness, and number density distribution, of the MMT survey and the full data set as a function of cluster-centric distance. Panel c: completeness as a function of r′ magnitude. Panel d: effect of the completeness correction calculated from panel c. The filled, grey histogram shows the g′−r′ colour distribution of all the potential targets. The solid magenta line shows the colour distribution of the galaxies with z estimate from the MMT data set corrected for completeness.