The MultiDrizzle Handbook
2.2 Benefits of Dithering
Dithering of HST observations is hardly new; the primary data acquisition modes of the GHRS and FOS involved both sub- and multi-diode offsets to obtain well-sampled data along the spectral dimension without gaps resulting from the presence of a few dead diodes. However, dithered observations in imaging-mode became routine only after the dramatic improvement in the HST optics that corresponded to the installation of COSTAR/WFPC2. Dithering often provides considerable benefits to the science program, specifically the following:
- Dithering can reduce the effects of pixel-to-pixel errors in the flatfield or spatially varying detector sensitivity.
- Integer shifts of a few pixels allow the removal of small scale detector defects such as hot pixels, bad columns, and charge traps from the image.
- Non-integral (sub-pixel) dithers allow the recovery of some of the information lost to undersampling by pixels that are not small compared to the point spread function.
The third point is particularly important in the case of HST imaging, since nearly all imaging instruments on HST are unable to take full advantage of the resolving power of the optics. This is because the instrument designers had a choice between fully sampling a small field of view, or using coarser sampling on a larger field. Dithering was particularly important when WFPC2 and NICMOS were the primary imagers on HST. The width of a WF pixel on WFPC2, at about 0.1 arcseconds, is already comparable to the full-width at half-maximum of the optics in the I-band and substantially exceeds it in the blue. The NICMOS camera 3 detector similarly undersamples the image over much of its spectral range. While the ACS/HRC does adequately sample the PSF at optical wavelengths, this comes at the cost of a drastically reduced field of view (1/50th the area of ACS/WFC). Even the ACS WFC, and the WFC3 UVIS and IR channels, have pixels comparable in width to the full-width at half maximum (FWHM) of the PSF, where ideally one would like a minimum two samples per FWHM for the full recovery of the image resolution. By dithering, one can recover these "missing" samples; however, one cannot completely undo the small blurring produced by a larger pixel. Nonetheless, dithering does substantially improve the final image quality, while simultaneously allowing improved removal of detector defects.
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