Flux-related colour–colour space for all arcfinder detections (background intensity), previously known arcs in the field for which we derived the photometry (red points), and region within the red lines that was used to select the final arcs sample. The white squares (a) are the candidates detected by the arcfinder, the white asterisks (b) are those segmented by the arcfinder, but rejected by its geometrical filters, and the cyan void squares (c) are those found purely by visual inspection. The latter are the most difficult sources and are scattered in a very wide area, far from most of the other sources, because of their uncertain photometry.