Synthetic spectra for the three cases considered in Sect. 2.3 corresponding to (top) few-lines spectrum, (middle) emission-dominated spectrum, and (bottom) absorption-dominated spectrum. The gray lines show the spectra with a 15 K noise, while the black lines show the spectra with 11 K noise. The red line together with the shaded yellow area correspond to the continuum level (plus noise) as determined with the sigma-clipping method.