Left panel: deviations of the FRs measured in all training spectra for the strongest metallicity relation (see Table A.2) at different resolutions with respect to the FRs computed at the original resolution of 45 000. In contrast to Fig. 9, we note that there is a non-linear relation between FR and [Fe/H], which is why we illustrate only the FR- and not the [Fe/H] deviations. The colored surface resembles the best-fit model to the data according to Eq. (14), where the color indicates the magnitude of the correction to be applied (see color bar on the right). Red points and their connecting line denote the track of the corrections to the FRs measured in the solar spectrum at different resolutions. Upper right panel: portion of the solar spectrum at resolutions of 45 000, 40 000, 30 000, 20 000, 10 000, 7500, 5000, and 2500 (dark- to light-colored). The color-coding refers to the same FR corrections as in the left panel. Blue vertical bars indicate the wavelength ranges from which the FR is computed. Lower right panel: derived [Fe/H] for the Sun from uncorrected (gray) and corrected (red) FRs using Eq. (10) and only the strongest metallicity relation shown in the upper right panel.