A number of dither patterns are available for ACS that automatically shift the target pointing between exposures. The size of the shifts depends on the purpose of dithering between exposures. It is useful to distinguish between mosaicing
. Mosaicing is done with the aim of increasing the area covered by a particular set of exposures, while providing a seamless joining of contiguous frames. Dithering is done for a variety of reasons, such as:
Patterns have been defined that easily implement mosaicing and dithering. These patterns allow exposures to be automatically associated in calacs
pipeline processing with the following restrictions: only exposures obtained within a single visit and exposures whose cumulative offset is under the ~100 arcsecond guide-star limitation can be associated. The latter condition includes the dither patterns for all three cameras, the SBC mosaic patterns, the 2-point ACS-WFC-MOSAIC-LINE
pattern, and all patterns designed with POS TARG
s. These are described in detail on the ACS Dither webpage
The plate scale for the WFC varies across the FOV by about
±5% due to distortion, so a one pixel dither near the center will be 0.95 or 1.05 pixels near the corners. For this reason, dither patterns should strike a balance between being large enough to reject detector artifacts, and being as compact as possible to maintain the integrity of the pattern over the entire field of view. Large displacements will have varying sub-pixel properties across the image.