The flat field response is uniform within a few percent, with the exception of a manufacturing pattern defect which generates a 3% reduction in QE once every 34 rows. ... Figure 4.4: WFPC2 CCD Flat Field. The WFPC2 CCDs have an intrinsically uniform flat field response since they are not thinned, so there are no large-scale chip non-uniformities resulting from the thinning process. ... The WFPC2 CCD flat fields show an overall pixel-to-pixel response having <2% non-uniformity. ...