Left: portion of the stellar spectrum of several T_eff calibrators (see text for details) with similar metallicity (from −0.07 to +0.15 dex), but different effective temperature: 3850 K (purple), 3701 K (blue), 3646 K (green), 3520 K (orange), and 3176 K (red). Right: portion of the stellar spectrum of several metallicity calibrators (see text for details) with similar effective temperature (3450−3550 K), but different metallicity: +0.06 dex (purple), +0.01 dex (blue), −0.11 dex (green), −0.20 dex (orange), and −0.37 dex (red). For the sake of clarity, an offset of −0.40 in flux was applied between the spectra.