Subtraction of the strong and only partly resolved airglow band O₂(a-X)(0–0) (cf. Fig. 8) for several time intervals between the object and the sky spectrum. The upper panel shows the band in the input object (black) and sky spectra (see legend for colour and line type). The lower panel displays the corresponding sky-subtraction residuals. Note that the ordinates are different in both panels. The two strong uncorrected lines in the centre and the left margin of the figure were caused by instrumental defects. Longwards of 1.275 μm, the O₂ lines are blended with OH(8–5) lines.