Left panel: example of DOLORES/TNG simulated mask. Right panel: example of OSIRIS/GTC simulated mask. See text for details. Black and green dots represent cluster members outside or inside the telescope field of view, respectively. Black rectangles are the slitlets, and the red points the line-of-sight velocity measurements (the symbol size is proportional to the mass of each galaxy with respect to the most massive galaxy in the mask). The circle in the dotted line represents the projected R₂₀₀ radius.