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 caused by a manufacturing error in producing the CCDs; there was a 0.5µm overlap between adjacent 1024x0.5µm raster scans during the construction of the masks used to fabricate the chips. It is identical in all CCDs. The net effect is that every 34th row on the CCD is approximately 3% too narrow. Photometry of point sources imaged onto these defects will be affected, since the error conserves counts, while flat fields (which are designed to produce a uniform image from a uniformly illuminated target) will effectively multiply the counts in these rows by 1.03. In applications requiring precision photometry across a wide field, it may be useful to correct the images for this flat field effect before performing photometry. There is also an astrometric offset of approximately 3% of the pixel height (0.003" in the WFCs) every 34 rows. Anderson and King (1999) present a nice discussion of these effects.

WFPC2 flat fields also include instrumental effects such as vignetting and shadowing by dust particles, and illumination variations related to optical geometric distortion. For further discussion see Section 5.11.

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.