Spectrum of QSO J0003−2603 at z = 4.125 before (black) and after (blue) telluric corrections. Three spectral windows with strong telluric absorption are shown. In the middle panel the tickmarks above the spectrum indicate two Fe ii absorption lines, λ2586 and λ2600 Å, associated with the z = 3.390 DLA. In the bottom panel we note how the Mg ii emission line stands out in the corrected spectrum.