Part of the Haro 11 SiC 2A extended source spectrum plotted with binning 0.08 Å and night data only. The Milky Way C iii λ977 Å line is seen to be saturated, indicating that the zero flux level is accurately calibrated using the background model presented in this paper. In addition, the Haro 11 Lyδ line redshifted to 969.2 Å seems to be saturated at the line center. The Milky Way absorption Ly-lines are however affected with airglow emission, and cannot be used as a calibration tool for Haro 11 (here Lyγ at λ972.5 Å).