|Space Telescope Science Institute|
|ACS Instrument Handbook Cycle 23|
Dithering and CR-SPLITing more than the minimum recommended values tends to yield higher quality images with fewer residual detector defects, hot pixels or CR signatures in the final combined image. Dithering is recommended over CR-SPLITs since it allows the removal of both detector artifacts (hot pixels, bad columns, etc.) and cosmic rays. Unfortunately, splitting a given exposure time into several exposures reduces its signal-to-noise when the sub-exposures are read-noise limited.Broad-band and grism WFC exposures longer than about 300 seconds are usually background limited (backgrounds >20e-), while medium- and narrow-band images are read noise limited for all practical exposure times. Thus, the optimal number of sub-exposures is a result of a trade-off between completeness of the hot pixel elimination, CR-rejection, final image quality, and optimal S/N. A schematic flow chart of this trade-off is given in Figure 7.2. The main steps in this, possibly iterative, process are the following:
2. Determine the maximum number of acceptable residual CRs in the final combined image. This number depends critically on the scientific objective. For example, for a survey of distant galaxies or a globular cluster color magnitude diagram, a few residual CRs will not compromise the scientific output of the observations. In contrast, for a search for an optical counterpart of some radio or gamma ray selected object even one residual CR would not be acceptable over the region of interest. In this latter case, since we expect about ~4% to 7% of the pixels to be affected by CR hits during a one-orbit exposure on the WFC, the requirement that no pixel in the final drizzle stack be affected by CR hits would force one to use at least 4 sub-exposures. For an experiment in which the number of allowed false alarms is zero (e.g., a search for cosmological supernovae), observers may wish to further increase the number of sub-exposures.
4. Determine whether dithering is required. CR-SPLITs do not mitigate hot pixels, which result from CCD radiation damage and often persist for weeks if not indefinitely. If such features would critically affect the science, then dithering is required to remove them. Hot pixels currently cover approximately 1.8% of the WRC CCDs. For some imaging programs the spatial resolution provided by the WFC and the presence of some detector defects and hot pixels in the final image are acceptable. For such observations, dithering would not be required and one would simply split the exposure time for CR correction. For observations where several orbits worth of data are obtained with each filter, the best strategy is to observe using a sub-pixel dither pattern without obtaining multiple images at each position. There are now so many hot pixels across the WFC CCDs that we recommend the use of CR-SPLITs over dithering only when absolutely essential that the sub-exposures place the source of interest at exactly the same location on the detector.