The purpose of this plot is to guarantee that all flat field calibration data have an exposure level which is good enough to record the pixel-to-pixel gain variations.
Check the other plots of this group for data from any measured flat field, sorted over time ("ALL") or over wavelength ("per WLN").
The data are checked against upper and lower limits. The lower limit is the most important one. From the CCD trending plot (no. 1), it follows that a single flat must have a minimum flux level of roughly 20.000 ADU at the fibre peak, in order to be dominated by the pixel-to-pixel gain pattern rather than by photon noise. From the topology of the flat frame signal, it follows a corresponding mean_raw value of about 7000 ADU. Assuming three input flats creating a single master fiber flat, we derive an approximate lower limit of 2000 ADU. This value is marked as lower threshold in the plot.
In the same way, the IFU flats require about 3000 ADUs on average.
The upper limit is derived from the saturation limit.