Properties of the X-shooter NIR-arm test data and quality of the sky subtraction by skycorr as a function of time. The time difference Δt is given relative to the object spectrum to be corrected for sky emission (2011/12/25 00:59:05 UT). The left, middle, and right panels characterise the sky data of the same night, about six nights before, and about one year before the object reference exposure, respectively. From top to bottom, the panels display a) the airmass, b) the summed intensity of two airglow lines of the OH(3–1) band, c) the ratio of the same lines tracing relative B-group variability, d) the ratio of the same rotational line in two different OH bands tracing the relative A-group variability, e) the ratio of two OH lines, which can only vary by effects unrelated to airglow, f) the sky continuum at 1.6 μm relative to the corresponding measurement in the uncorrected object spectrum, g) a similar ratio for 2.1 μm, h) the ambient temperature in K, and i) the rms of the sky-subtraction residuals for line pixels relative to the mean line peak flux. The latter is shown for two different minimum degrees of the Chebyshev polynomial for the improvement of the wavelength solution by skycorr. In the middle panel of i), the effect of an initial shift of the wavelength scale by one pixel is also plotted (dotted line).