Normalization procedure for the CI, data for the field FDS#13 are shown. Left panel: concentration index CI = mag_{4 pix} − mag_6 pix versus the uncalibrated a-stack magnitude. For the sake of clarity, only the brightest magnitude range is shown. Blue dots refer to the full sample, red symbols to candidate compact sources used to derive the median CI factor for normalization. Right panel: same as left panel, but over a larger magnitude range and after normalization to the median CI of bright point-like sources (red dots). Point-like sources candidates are aligned along the sequence parallel to the x-axis, around CI_n ∼ 1 (green dot-dashed line).