Effect of the calibration procedure on the CaHK magnitudes. Left: maps of the CaHK offsets between the uncalibrated (top) and calibrated (bottom) Pristine catalogues and the Gaia CaHK_syn magnitudes (the color codes CaHK_uncalib − CaHK_syn and CaHK_calib − CaHK_syn, respectively) for run 17Am05. Right: histogram of a random sampling of the residual (∆CaHK = CaHK_calib − CaHK_syn) for 1% of stars in the calibration (δCaHK < 0.05; blue) and validation (δCaHK < 0.015; orange) samples.