Difference z_X − z_opt for the 35 sources with spectroscopic or photometric redshift plotted as a function of the ΔC_stat value. Red circles (triangles) are for clusters whose photometric (spectroscopic) redshift is found in the literature through a match with the NED, while the magenta squares are for clusters whose photometric redshift has been computed by us with a dedicated TNG program or with VLT archival image (in one case). Crosses corresponds to z_X values rejected after a visual inspection because a secondary minimum in C_stat is found close to the minimum (see text for details). Error bars on z_X and z_opt correspond to 1σ.