2D spectra. Panels (a), (b), and (c): Red part of the spectrum around the Hα and [NII] 6548 and 6583 lines. Panels (d) and (e): Blue part of the spectrum around [OIII] 5007. Panel (a): Raw spectrum containing the night skylines. Panel (b): Same as panel (a), but the night skylines are subtracted. Panels (c1) and (c2): Position of the sky lines is shifted by -0.5 and + 0.5 Å with respect to panel (b). The white dotted rectangle in panels (c1) and (c2) shows the position of the Hα and [NII] 6583 lines as indicated by panel (b). Panel (d): Raw spectrum showing mainly two sodium night skylines. Panel (e): Same as panel (d). The night skylines are cleanly subtracted, but the [OIII] 5007 line is not visible in the 2D spectra.