Spectroscopic redshift distribution of the objects in our final spectroscopic catalogue. Cluster members (i.e. lying in the redshift range from z = 0.211 to z = 0.251) are in blue, whereas foreground and background objects are in orange and green, respectively. Multiple images are depicted in red. The vertical black line locates the redshift of the galaxy cluster. The insert shows the cluster members selection illustrated in Sect. 3.1. The dashed red line identifies the best-fit Gaussian distribution, whereas the vertical black dotted lines define an interval of ±3σ_z around the median cluster redshift.