Mean wavelength residuals as a function of airmass for six observations of NGC 7099 and one additional sky frame. The black data points reflect the uncorrected weighted mean of all sky line residuals as explained in the text and shown in Fig. 17. Correcting the wavelength solution of each observation by means of Eq. (11) removes the bias (blue data points). The error bars illustrate the uncertainties of the weighted mean and represent the significance of the observed trend with airmass. The airmass at which the AFC measurement was performed is indicated with the dashed red line.