Estimation of the galaxy magnitude completeness limit. The grey points are all galaxies on red sequence in the cluster radius. Red points are those with r_K20 >  3″ (that is, the cluster sample). Line a is the relation fitted through the grey points with the 1σ and −1σ deviation shown by the grey dotted lines. The surface brightness threshold for all 3C 129 galaxies on the red sequence is illustrated by line b. The completeness limit is given by the intercept of line b and the −1σ line. The corresponding surface brightness threshold and completeness limit lines for the bonafide cluster sample with r_K20 >  3″ are denoted by the solid grey vertical and red line, respectively.