Quality of the CaHK magnitudes. Bottom panels: CaHK magnitude uncertainties for stars in a representative sample of the Gaia-based CaHK_syn (left) and Pristine (right) catalogues. The colors code the G_BP – G_RP color of a given star and show the clear impact of a star’s color on the CaHK signal-to-noise in both samples. The sharp faint edge in the Gaia-based synthetic catalogue is driven by the DR3 G < 17.65 cut and the small number of fainter sources correspond to specific fainter targets included in the catalogue by the Gaia collaboration (De Angeli et al. 2023). The Pristine survey, on the other hand, has significantly higher signal-to-noise and only reaches photometric uncertainties δCaHK = 0.1 at 19.0 ≾ G ≾ 21.0. Top panels: histograms of star counts as a function of magnitude, with the line style tracking different samples, as labeled in the panels.