Upper panel: with the determined transfer function (Fig. B.1lower panel), one can calibrate the raw spectra (Fig. B.1upper panel). The individual spectra show a slight slope. T_sky is equivalent to the overall system temperature T_sys (which of course is a function of observational parameters as telescope pointing position, temperature, atmospheric conditions, etc). Lower panel: using the results from the upper panel, one can recover the T_cal spectrum. Note that the blue curve is different from the two other measurements. (For a discussion of this issue, see text.)