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. MPP operation also improves pixel-to-pixel uniformity because charge transfer is driven deep into the buried n-channel, away from the influence of Si-SiO2 interface states. The WFPC2 CCD flat fields show an overall pixel-to-pixel response having <2% non-uniformity. Figure 4.4 shows a portion of a WFPC2 CCD flat field obtained during quantum efficiency measurements at JPL. The image illustrates the excellent pixel-to-pixel uniformity of the Loral devices. The 34 row defect is clearly visible, and its amplitude of 3% serves to calibrate the gray scale.