Proper motion diagram of the central field of the Arches cluster. The circle denotes the selection limit for cluster members. Members with L′ detections are highlighted in green, while L′-detected non-members are shown in red. As already found in Stolte et al. (2010), all excess sources recovered in the cluster centre are proper motion members of the Arches. The right panel displays a zoom on the cluster selection and the median astrometric uncertainty for L-band detected members in the lower left corner.