Bisectors of Fe I 6173.3 Å measured at the solar disk center (μ = 1.0). The normalized intensity is displayed against the absolute air wavelength (lower x-axis) and the translated Doppler velocity (upper x-axis). The individual bisectors of 12 observational sequences are shown as blue curves with asterisks. The average bisector (black solid line) and its standard deviation (gray area) are given. The line bisector from the Fourier Transform Spectrometer (FTS, Neckel 1999) disk-center atlas is added in orange.