Spectroscopic confirmations with MUSE. Top panel: redshift distribution of the objects in the MUSE catalogue in bins of δz = 0.002. The magenta and blue histograms indicate, respectively, the ACT0102 cluster members (167 galaxies) and the foreground group (20 galaxies) used in our strong lensing modelling. The upper x-axis shows the line-of-sight rest-frame galaxy velocity with respect to the cluster member median redshift value of z = 0.8704. Bottom panel: redshift distribution of background single sources (i.e., corrected by image multiplicity) with δz = 0.2, where the overdensity at z ≈ 4.2 is clearly noted.