Overview of radio luminosity selection resulting from spectroscopic redshift selection, radio surface brightness limitations, and the correlation between radio and optical luminosity. The top panel compares radio luminosity distributions at ν_ref = 150 MHz of LoTSS DR1 RGs in the constrained part of the BORG SDSS, distinguishing between those with spectroscopic redshifts (green) and those without (grey) (We perform KDE on samples of log₁₀(L_ν(ν_ref)⋅(W Hz⁻¹)⁻¹) rather than of L_ν(ν_ref), and thus the probability densities on the vertical axis are dimensionless.). Radio galaxies with spectroscopic redshifts dominate (77%) the population. The bottom panel shows the two subpopulations in radio luminosity–redshift space, with RGs without spectroscopic redshifts marked (grey circles). We show uncertainty ellipses whose semi-major and semi-minor axes represent half a standard deviation along each dimension. One retrieves the top panel by collapsing the data on the horizontal axis.