Estimation of the relative error on the continuum level determination, depending on the molecular/continuum (M/C) ratio and continuum/noise (C/N) ratio. The black lines represent the M/N ratio. The typical ratios from the core #3 pixels are included in the dashed box. Each pixel is the average of the errors obtained from 20 synthetic spectra of 1000 channels with 100 random molecular Gaussian lines that follow a power law comparable to the observed data.