Top panel: χ²-surface as a function of the parameters ν and σ defining the spectrograph profile (Eq. (2)). This surface was obtained from comparing the observed () and simulated () quiet-Sun intensity profile of the Fe i line at 1564.8 nm. Bottom panel: FTS intensity profile of Fe i 1564.8 nm I_fts (crosses), simulated average quiet-Sun intensity profile obtained after applying the spectrograph profile to FTS data (solid line), and observed average quiet-Sun intensity profile from GRIS data in NOAA 12049 (filled circles; see also blue dashed rectangles in Fig. 1).