Blue data points show the FWHM (km s⁻¹) of Gaussian fits to unresolved telluric absorption lines in the VIS spectra, as a function of the airmass corrected DIMM seeing. The red data points show a subsample of measurements obtained for NIR spectra. The coloured lines show the corresponding linear fits to the data points. The coloured bounds contain 68 per cent of the best-fit probability mass. The red and blue dashed lines indicate the nominal resolution for the slit-widths used for the telluric standard observations. As can be seen, the effective resolution is in many cases superior to the nominal one.