Difference between synthetic magnitudes from the reference spectra and synthetic magnitudes from the XP spectra as a function of G magnitude (upper panels) and B_p−R_p colours (middle panels) for SIMBIO-SYS HRIC_PAN e STC_PAN. The thick horizontal line marks Δmag = 0.0. Points are colour-coded after the reference set they belong to, pink=SPSS, grey=CALSPEC, yellow=PVL. The mean and standard deviation of the magnitude difference after applying the correction (formula reported on the plot), for the 37 CALSPEC stars having G<10.5 mag, is reported in the top-right corner of upper panels. The correction is plotted in the upper panel if it is a correction as a function of G and in the middle panel if it is a correction as a function of B_p − R_p. The residuals before and after (i.e., difference between synthetic magnitudes from the reference spectra and corrected synthetic magnitudes from the XP spectra) the correction for CALSPEC stars are plotted as a function of extinction E(B-V) in the bottom panel with grey markers and black crosses, respectively. We adopted the E(B-V) values provided by the CALSPEC collaboration (https://www.stsci.edu/hst/instrumentation/reference-data-for-calibration-and-tools/astronomical-catalogs/calspec, data retrieved on September 5, 2024 (Bohlin et al. 2020)).