Spectroscopic redshift validation of a cluster using SPIDERS follow-up observations, as described in Sect. 4.1.3. Top-left panel: distribution of red-sequence galaxies in Xclass0842 as a function of their projected distance to the X-ray centroid and the velocity offset to the mean cluster redshift. Blue dashed lines indicate the standard deviation of the velocity distribution. Top-right panel: location of those members projected on the sky, with a similar colour-coding as in the previous panel. Lower-left histogram: redshift distribution of galaxies in the red sequence with an associated spectroscopic measurement. Lower-right histogram: distribution of members (in blue) and non-members (in grey).