Flat fields at different wavelengths are shown in figures
6, 7 and 8.
Large scale uniformity is very good at visual and UV wavelengths, with
peak to peak variations of 3%. Around 1
the non-uniformities
increase to 10%.
At the extreme corners and edges, sensitivity
goes down as the chip is not thinned here. An X-like structure is seen from
corner to corner - this is most probably light reflected off the concave border
to the thinned area at the corners.
Specks a few pixels wide having reduced sensitivity are scattered over the entire imaging area. Setting a threshold 3% lower than the median sensitivity, 1% of the area is affected by the specks. The central sensitivity typically goes down by 20%, but a few areas goes much lower. All of the specks can be corrected by flat-fielding.
Charge traps and bad columns can be found by comparing a flat field exposure at very low illumination level to a well exposed one, as illustrated in figure 1. In the table below, the coordinates of defects found are listed. Note that the number of pixels affected by a trap depends on the illumination. The actual trap location is the smallest Y-coordinate of the area.
Figure 1:
Low illumination level flat field properties, as a mean of identifying
charge traps.
Lower left: Flat field at an illumination level of 74
/pixel.
Grey scale cuts are 
5% of median level.
Lower right: Flat field at an illumination level of 40.000
/pixel.
The tilted square and central blob structure are caused by stray light in
the setup.
Upper left: Ratio of the two flat fields. The bright vertical lines
show areas affected by low level traps.
Grey scale cuts are 
15% of median level.
Upper right: Different cuts applied to the ratio image, isolating traps