This plot trends the contamination measurements. The image flat with the highest exposure time is taken, its flux is measured in the five windows marked in plot 2, and normalized to the center (meaning the central box has always intensity 1 in plot 3). If contamination builds up, the relative efficiency will go down. The GIRAFFE CCD has a typical contamination pattern, with the lower left corner showing the strongest effect. Its value is plotted red and emphasized by the connecting line.
A decontamination procedure should be applied when the red data start to approach the value 0.9.
Other QC parameters related to detector properties are trended in the same group.