Distribution of galaxy clusters with mass and redshift as it will be detected by the eROSITA instrument during its four years of all sky survey based on the mass function by Tinker et al. (2008) and on the scaling relations by Reichert et al. (2011). The colour bar indicates the number of galaxy clusters in the individual bins in units of log ₁₀ and the cluster mass is considered in units of log  (M/M_⊙). We assume that a minimum number of η_min = 50 photons is necessary to identify a cluster and effectively apply a lower mass cut to exclude low mass galaxy groups.