Flat fields at different wavelengths are shown in figures 6, 7 and 8.
The large scale uniformity is very good in the middle of the optical band, with variation of about 1% over the field. As the CCD is nearly transparent at the longest detectable wavelengths, thickness variations and internal reflections makes the non-uniformities increase to typically 5%, with a single deviation of up to 20%. In the near-UV, a 5% diagonal gradient in sensitivity is seen. At the extreme corners and edges, sensitivity goes down as the chip is not thinned there.
Specks a few pixels wide having reduced sensitivity are scattered over the entire imaging area. Setting a threshold 5% lower than the median sensitivity, 1% of the area is affected by the specks. The central sensitivity in the specks typically goes down by 20%, but a few areas goes much lower. All of the specks can be corrected by flat-fielding. There appears to at least two types of specks. Some are probably an opaque deposit on the surface, like the scratch and dust-like structures, others are probably defects in the coating - more on these later.
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 level. The actual trap location is the smallest Y-coordinate of the area.
If the CCD is warmer than -90^
C, a group of about 10 warm columns will
appear around column x=216.
Figure 1:
Low illumination level flat field properties, as a mean of identifying
charge traps.
Lower left: Flat field at an illumination level of 71
/pixel.
Grey scale cuts are 
300% of median level.
Lower right: Flat field at an illumination level of 71.000
/pixel.
Grey scale cuts are 
10% of median level.
Upper left: Ratio of the two flat fields.
Note that nearly all low sensitivity spots are not seen here, indicating
that flat-fielding of these is possible and that low-level linearity
is good.
The vertical lines show areas affected by low level traps.
Grey scale cuts are 
200% of median level.
Upper right: Deviating points in the ratio
image, affected by traps.