Left panel: synthetic spectra for different 3D model red giant atmospheres in the wavelength range 4700−5300 Å. The intensity maps corresponding to this filter have been computed assuming a top-hat filter (dashed line). This wavelength range corresponds to the VEGA instrument (Mourard et al. 2009) mounted on CHARA with a spectral resolution λ/Δλ = 5000. From the bottom: the black curve refers to the simulation with log g = 2.2 and [Fe/H ]  = 0.0 and no offset applied, the red curve to log g = 2.2 and [Fe/H ]  = −1.0 with an offset of 1, the green curve to log g = 2.2 and [Fe/H ]  = −2.0 with an offset of 2, the light blue curve to log g = 2.2 and [Fe/H ]  = −3.0 with an offset of 3, and the dark blue curve log g = 1.6 and [Fe/H ]  = −3.0 with an offset of 4 (see Table 1). Right panel: transmission curve (dashed line) of the FLUOR (2.14 ± 0.26 μm) filter (Mérand et al. 2006) mounted on CHARA. Colors and offsets are the same as in left panel.