Calibrated spectrum of APM 08279+5255 before (green dashed line) and after dereddening (black solid line), with corresponding 1σ uncertainty (gray band) – see Sect. 2. Superimposed to the data, the composite quasar template obtained by matching the Vanden Berk et al. (2001) and Glikman et al. (2006) templates around λ ~ 3000 Å in the restframe (red line) is shown. The continuum level is marked as a black dotted line with slope − 1.54. A residual telluric absorption is still present between λ ~ 17 500 Å and 20 000 Å (cyan shaded area), hence this spectral interval is excluded from the present analysis.