2.5 Dithering Considerations

The ACS Team recommends that observers dither (or offset) their observations to mitigate the effects of hot pixels, cosmetic defects, and cosmic rays in their combined images. Dithering allows improved sampling of the point spread function and yields better images than are possible with the (generally not recommended) CR-SPLIT option, which does not remove hot pixels, permanent cosmetic defects (e.g., bad columns), or the gap between the WFC CCDs. It is noteworthy that hot pixels now contaminate 2% of the WFC detector, though >90% of these are stable and can be reliably dark-subtracted (ACS ISR 2017-05). For more on hot pixels, see also ACS ISR 2022-07, and references therein.

Dithering can be performed in two ways:

1. explicit positional offsets between exposures via POS TARG instructions; or
2. flexible pre-defined dither patterns that can be nested to implement different pixel subsampling strategies.
   

Both methods yield associations of images for pipeline data processing. Currently available pre-defined dither patterns and their recommended uses are described on the ACS Dither webpage. Section 7.4 also discusses dithering in more detail. In cases where observers are using ACS/WFC and WFC3 in parallel, there are predefined dither patterns designed to optimize the sampling in both detectors to the best extent possible. See ACS ISR 2023-04 for details.

The ACS Team at STScI is available to help observers select dither patterns that best suit their science goals. Because STScI wishes to maximize the legacy value of HST observations, observers who choose not to dither their ACS/WFC exposures must provide a justification in the Description of Observations section of their Phase I proposal.