Difference of synthetic Hα magnitudes from HR and XP spectra for SPSS, PVL, and selected NGSL as a function of G_BP − G_RP colour. In this case, the width of the TC adopted for synthetic photometry on HR spectra is kept fixed (FWHM = 9.0 nm), while the width of the TC used for synthetic photometry on XP spectra is varied from FWHM = 12.0 nm to FWHM = 9.0, 6.0, 3.0 nm, from top to bottom. The various panels are labelled according to the ratio between the FWHM of the XP and HR TCs.