Classification of images based on the value of Δ_x. The top plot shows Δ_x for 19 000 UBr measurements of HD 138690 (γ Lup) in the Sco I field. The two horizontal lines indicate the high (Δ_high = 36 pix = 0.75X, X = 48 pix) and low (Δ_low = 12 pix = 0.25X) cut-off thresholds. The three images shown below correspond to three cases, from left to right: useful image (Δ_low< Δ_x = 24.91 pix < Δ_high); image with overlapping profiles (Δ_x = 2.99 pix < Δ_low); image with profiles too far from each other (Δ_x = 37.20 pix > Δ_high).