The median of the 21 central rows of the master flat is taken and a cross section over one order is extracted. (see plot tutorial for details). For flats in slit mode an analysis region is determined exluding both edges. For flats in IFU mode the analysis region is confined to the central slice.
Boxes 1 and 4 show the rms of the analysis region.
The analysis region is fit by a linear function
y = a + b * x
where b represents the illumination gradient over the slit.
This QC parameter is given as gradient in box 2 for slits and in box 5 for the central
slice in IFU flats.
The uncertainty of the gradient b, called grad_err, is given in boxes 3 and 6.
Thresholds are based on the experience gained during the 2017-02 events in the UVB-arm