This plot shows the relation between SNR (called 'mean_s2n' in the database) and average counts (mean_reduced) for the UVES IDPs.
Each individual plot show both the stacked products (whenever the OBs could be stacked) and the single spectra.

The two QC parameters measure the quality of the reduction process: the higher the flux, the higher is the expected SNR if the noise is dominated by photon noise. In case of stacking, the SNR of the products roughly scales with sqrt(N) where N is the number of identical input spectra.

We have selected reference settings 346 and 437 for the BLUE arm (CCD), and 580 and 860 for the RED arm. We have selected data taken in 1x1 binning and without additional optical components (like SLICER or Iodine cell).

The QC parameters plotted here are averages across the whole spectrum. They depend somewhat on the slope of the spectra and therefore on the nature of the targets. This might explain the observed spread in the relations.