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Spatial variations in QE ratios are comparable to expected statistical fluctuations for FI CCDs except for bad or hot columns or pixels and border effects (especially the shadows of frame store covers). These superpixel ratios are generally outliers in the histogram of 1024 QE ratios, and only weakly affect the Gaussian fit. These effects may be seen in Figure 4.64. Columns darker than surrounding regions correspond to either bad columns in the flight CCD or hot columns in the reference CCD, while lighter columns typically correspond to bad columns in the reference CCD. The bright row at the bottom of the images is believed to be due to unequal shadowing by the frame store array covers on the two CCDs.
Larger spatial variations are observed in BI CCDs, and are believed to be caused by relatively high CTI (charge transfer inefficiency), exceeding 25% at higher energies. Some provision also needs to be developed to characterize this spatial nonuniformity in the two flight BI CCDs (S1 and S3). These variations affect the quality of the Gaussian fit to the histogram of QE ratios.