CCD Performance

4.7 Flat Field Response

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. This pattern defect is identical in all CCDs. It probably creates an astrometric offset of approximately 3% of the pixel height (0.003" in the WFCs) every 34 rows. More precisely, there was a 0.5 micron overlap between adjacent 1024x0.5 micron raster scans during the construction of the masks used to fabricate the chips. Photometry of point sources imaged onto these defects will be affected, and it will be better to correct flat fields for these rows for such applications. WFPC2 flat fields also include instrumental effects such as vignetting and shadowing by dust particles.

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.

Figure 4.4: - WFPC2 CCD Flat Field