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Advanced Camera for Surveys Instrument Handbook for Cycle 20 > Chapter 7: Observing Techniques > 7.4 Patterns and Dithering

7.4
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 and dithering. 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 TARGs. These are described in detail on the ACS Dither Web page:
http://www.stsci.edu/hst/acs/proposing/dither.
The plate scale for the WFC varies by about 5%, 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.
In addition to the plate scale variation associated with the significant ACS geometric distortion, there can also be a temporal variation of overall image alignment. Some CR-SPLIT images taken during SMOV (SM3B) testing, in which the two components were separated by the scheduling system across orbital occultations (about a one hour gap), showed registration differences of about 0.5 pixels corner-to-corner. Thus, to combine multiple images to create oversampled images at the resolution ACS is capable of providing, the user may need to allow for the general problem of combining distorted, misregistered images. A number of tools are available to help users align and combine dithered data. For information on how best to reduce dithered ACS data we recommend users obtain the MultiDrizzle Handbook available at:
http://stsdas.stsci.edu/multidrizzle.

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