|Space Telescope Science Institute|
|ACS Instrument Handbook Cycle 19|
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 detectors artifacts (hot pixels, bad columns, etc.) and cosmic rays. Unfortunately, splitting a given exposure time into several exposures reduces its signal-to-noise when an image is read noise limited. WFC images longer than about 500 seconds are background limited, while shorter exposures and narrow band images are read noise limited for all practical exposure times. Thus, the optimal number of CR-SPLITs and dithering positions 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 CR 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 CR 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 image is 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 consider using at least twice the number of sub-exposures required to formally avoid coincidences. Note also that given the large number of pixels in the WFC even a few thousand residual CR hits would correspond to only a small fraction of the total number of pixels. In general, the number of pixels affected by coincident CR hits for a given total exposure time and number of sub-exposures N will be:
3. Determine whether dithering is required. CR-SPLITs do not mitigate hot pixels, which result from CCD radiation damage and which may persist for weeks if not indefinitely. If such features would critically affect the science, then dithering is required to remove them. 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. Since each CR will now influence more than one output pixel the requirement on the number of separate exposures is more stringent than in the simple CR-SPLIT case. If the total exposure time with each filter is short, one will have to compromise between S/N and image quality. In general, dithering with sub-pixel steps increases the number of individual exposures required to eliminate CR hits. Given that the geometric distortion of WFC makes any dithering step non-integer somewhere in the field of view (unless the dither steps are very small, < 2 pixels), the size of the high image quality field of view also comes into play. If the high quality area is small, one may make do with integer pixel dithers. In this case a few CR-SPLITs may be obtained at each dithering position and the combined images may then be combined together using drizzle or multidrizzle. On the edges of the field the CR-rejection quality will be lower than in the field center. A minimum of 4 images for a two position dither, and 8 for a four position dither are then required.
4. Once the required number of individual exposures has been established on the basis of CR rejection and dithering requirements, the observer will need to verify whether the resulting read-out noise affects the achieved S/N.