The MultiDrizzle Handbook
3.4 Image Fidelity After Drizzling
In the drizzle algorithm, the weight of an input pixel in the final output image is independent of its position on the chip. Therefore, if the dithered images do not uniformly sample the field, the center of light in an output pixel may be offset from the center of the pixel, and this offset may vary between adjacent pixels. Furthermore, the distortion present in the imaging instruments on board HST produces sampling patterns that are not uniform across the field, due to the changing pixel size. This directly impacts the uniformity of the output PSF.Figure 3.4: PSF Resulting from Non-uniform Sampling
PSF is taken directly from the HDF-N F450W drizzled image and shows substantial variation about the Gaussian due to the effects of non-uniform sampling, as well as possible additional charge-transfer effects in the CCD.
Figure 3.5: PSF of a Bright Star with Uniform Sampling
PSF from the HDF-N (Fruchter and Hook 1997) is a bright star taken from a deep image with a nearly perfect four-point dither, and clearly shows the improvement in the PSF resulting from the more uniform sampling.
This effect is seen in the HDF-N images, where some pointings were not at the requested position or orientation. Figure 3.4 and Figure 3.5 show two PSFs compared with best-fitting Gaussians. Although Gaussians are only a crude approximation to the real PSF, they nevertheless suffice to illustrate the point of this particular example. The upper PSF is taken directly from the HDF-N F450W drizzled image, and displays a substantial amount of variation about the Gaussian fit. In contrast, the lower PSF is a bright star taken from a deep image with a nearly perfect four-point dither, in which the uniform sampling has produced a much smoother PSF. (Note that the difference in the apparent widths of the PSFs is due to the use of larger output pixels in the second image than in the HDF-N: 0.05 vs. 0.04 arcseconds).
Changes in PSF can also result from other problems, such as charge transfer errors in the CCD (Whitmore & Heyer 1997; Heyer 2001). Generally, however, these variations are likely to be less noticeable than effects due to non-uniform subsampling of the PSF.
Space Telescope Science Institute
Voice: (410) 338-1082