Bottom left: field density-subtracted, completeness-corrected CMD. Each colour encodes a galaxy overdensity (purple) or underdensity (orange), expressed in count per arcminute square. We restricted the magnitude to H_E ≤ 22.25. As in Fig. 2, the locations of the red sequence at different redshifts are indicated by grey lines, and the location at z = 1.74 is indicated in pink. On the z = 1.74 red sequence, H_E = 22.25 corresponds to z = 24.69 and to a completeness level of 76%. The cyan line on the very top right on the panel indicates the 50% z completeness limit and the dashed green line shows the location of the red sequence/blue cloud boundary. The proto-BCG photometry is marked by a cyan dot. Top left: count of member galaxies as a function of H_E-band magnitude, after subtraction of the background level. The solid green and black step lines indicate the background-subtracted counts before and after the application of the completeness correction, while the shaded regions indicate count uncertainties. Bottom right: background-subtracted members count as a function of colour with and without the completeness correction.