2.2 Dithering in Cycle 17
The ACS-WFPC2 Team has long recommended that observers dither (or offset) their observations to remove hot pixels, cosmetic defects, and cosmic rays in the combined data. Dithering allows for optimal sampling of the point spread function and provides substantive improvements over the use of CR-SPLITs, which do not remove hot pixels or other permanent cosmetic defects (e.g., bad columns or the WFC interchip gap). Dithering can be achieved in two ways: 1) By using exposure-to-exposure position offsets specified explicitly by POS-TARGs, or 2) By using flexible predefined dither patterns. Both POS-TARGs and dither patterns now result in associations, and dither patterns can also be nested to achieve various pixel sampling strategies for a wide variety of science goals. Currently available dither patterns within APT and their recommended uses are described on the ACS Dither Web page:
We strongly encourage observers to dither their ACS observations. This is particularly important for cycle 17 given that we cannot predict the impact of 18 months of continuous radiation damage without periodic anneals. Observer who choose not to dither their ACS WFC and HRC imaging exposures should provide sound scientific justification for not doing so in the Phase I Proposal Description of Observations. The choice of dither pattern is left to the discretion of the observer since it may depend upon the science goals of the program. The ACS-WFPC2 Team is ready to assist observers who need help in selecting the appropriate pattern for their observations.
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