An example of how we estimate the Guess spectrum for the C+ line of Arp 220 (obsID 1342202119). We compute the histogram of the normalized χ² of each individual spectral scan with respect to the median spectrum (top panel). The blue line shows the well behaved scans and the red line shows those to be rejected. The Guess spectrum (bottom panel; green line) is the result of co-adding the well behaved scans (blue lines). The scans with χ² deviating more than 3σ from the median value are discarded (red lines).