Purity and completeness contours to identify the optimal location of extent−extension-likelihood plane for the baseline 80 ks simulation. Left: black purity contour lines are used to identify the source contamination levels, the orange line represents a 0.44 sources per image contamination rate, which corresponds to one source per deg² false detection rate. Right: completeness contour is then used to find the location to maximize the detection probability of galaxy groups. The orange line is the same as in the left panel, and the orange star represents the selection criteria to distinguish between point-like and extended sources, at ∼93% completeness level.