The best-fit spectrum, convolved with the instrument response. It is shown in the top panel as a one-dimensional spectrum and as a clean two-dimensional array in the second one. The third panel shows a 1σ noise two-dimensional array in the CCD area corresponding to the afterglow spectrum, as described in Sect. 3.2.2. The fourth panel shows one realisation of the CCD noise plus the best-fit spectrum, which can be directly compared to the real data at the bottom panel. The contrast scale is the same in the two last panels, but not in the previous ones.