This plot shows the median counts in the brightest of five raw input flats.
The QC parameter is monitored for the left beam
and for the four broad band filters in the common filter wheel: B_Y, B_H, B_J and B_K, 
separately. The dual beam filter wheel is 'CLEAR' in classical imaging mode. 
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The bi-variate distribution in the B_Ks filter values originate from different
templates.
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<bf>New 2018-10-16:</bf> The plot shows the counts in one plane of the raw frame cube corresponding to NDIT=1. 
For NDIT=40, the total counts in the brightest raw flat is 40 times the counts of one plane of the raw frame cube. 
To achieve a S/N-ratio = 200 in the brightest raw flat, about 1000 ADU (new threshold) must be achieved in a single plane.
The S/N ratio in the final master flat is not checked for.