Left: detection likelihood (det_ml) as a function of total detected source counts (PN detector only) for point sources (open circles) and the detected clusters (full circles). Clusters are color coded by their extent (beta model core radius). The dashed red line shows the best fit linear relation in the det_ml – counts plane for point sources. The solid line shows the same relation for extended sources (with slope fixed to the point-source fit, points weighted by their counts error). Typically, an extended source has to have 2.4 times more counts than a point source to be detected at the same det_ml value. See Sect. 5.2 for details. Right: luminosity in the 0.5−2 keV band (object rest-frame) for the present cluster sample as a function of redshift. The line shows the luminosity of a cluster with a measured flux of 1 × 10¹⁴ erg s^-1 cm^-2 (unabsorbed, observer rest-frame).