7.4 Patterns and Dithering

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:

• Better removal of detector blemishes
• Straightforward removal of hot pixels
  
• Improving the PSF sampling
  
• Improving the photometric accuracy by averaging over flat fielding errors
  
• Obtaining a contiguous field of view for the WFC (filling in the interchip gap)

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 strategies webpage. More general information about pointings and patterns can be found in Chapter 7 of the HST Phase II Proposal Instructions, and more specific information about ACS dither patterns is included in Section 7.2 of the HST Phase II Proposal Instructions.

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. For parallel observations (using WFC3 in particular), dither patterns that optimally sample the pixel-phase in one instrument do not necessarily have optimum pixel-phase sampling in the other instrument due to different geometric distortion properties between instruments. To address this issue, a library of co-optimized ACS-WFC3 parallel observation dither patterns are available. Additional details can be found in ACS ISR 2023-04.

In addition to the plate-scale variation associated with the significant ACS geometric distortion, there can also be a temporal variation of the 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 pixel 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 check the DrizzlePac website.