View of the C3VO observations of the PCl J1001+0220 proto-cluster at z ∼ 4.57. Left: layout of the two masks observed with DEIMOS as part of the C3VO survey in the vicinity of the PCl J1001+0220 proto-cluster against the backdrop of a two-dimensional overdensity map generated using the methodology described in Sect. 2.4, though here the overdensity is calculated over the entire redshift extent of PCl J1001+0220 indicated at the top of the plot. The two completed DEIMOS slitmasks are outlined in black. The dashed circle denotes R_proj = 2 Mpc centered on the barycenter of the detection in the overdensity map. The highest priority DEIMOS targets (i⁺ < 25.3 and 4.44 ≤ z_phot ≤ 4.70) in the vicinity of PCl J1001+0220 are shown as filled dark green circles. Galaxies with high-quality z_spec measurements from VUDS and from our new DEIMOS observations within the redshift range of PCl J1001+0220 are shown as purple squares and blue stars, respectively. Galaxies with high-quality z_spec measurements from the DEIMOS observations at z > 4 but not within the redshift range of PCl J1001+0220 are shown as white crosses. Galaxy overdensity contours indicate levels of ∼3, 4, 5, 6, 7, 8σ_NMAD in excess of the background. Right: redshift histogram of all objects with secure z_spec measurements from Keck/DEIMOS observations. The blue hatched histogram shows the z_spec distribution of all targeted galaxies with a secure z_spec, while the gray solid histogram also includes those galaxies that serendipitously fell on in the area subtended by the slits in the two masks. These DEIMOS observations add considerably to the high-redshift portion of the sample presented in this study, especially at the high-density end.