Simulated and detected clusters population as a function of the input flux and size on the sky. The figures refer to areas of the eRASS1 simulation covered by an exposure between 150 s and 400 s. The blank spaces contain no input clusters. Top panel: number of simulated clusters in the flux–R_500c space. Central panel: fraction of simulated clusters that is detected by eSASS, either as extended or point source. Bottom panel: fraction of simulated clusters that is only detected as extended.