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ACS Instrument Handbook Cycle 23
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Advanced Camera for Surveys Instrument Handbook for Cycle 23 > Chapter 7: Observing Techniques > 7.5 A Road Map for Optimizing Observations

7.5
Dithering and CR-SPLITing more than the minimum recommended values tends to yield higher quality images with fewer residual detector defects, hot pixels or CR signatures in the final combined image. Dithering is recommended over CR-SPLITs since it allows the removal of both detector artifacts (hot pixels, bad columns, etc.) and cosmic rays. Unfortunately, splitting a given exposure time into several exposures reduces its signal-to-noise when the sub-exposures are read-noise limited.
Broad-band and grism WFC exposures longer than about 300 seconds are usually background limited (backgrounds >20e-), while medium- and narrow-band images are read noise limited for all practical exposure times. Thus, the optimal number of sub-exposures is a result of a trade-off between completeness of the hot pixel elimination, CR-rejection, final image quality, and optimal S/N. A schematic flow chart of this trade-off is given in Figure 7.2. The main steps in this, possibly iterative, process are the following:
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Figure 7.2: Schematic flow-chart of the CR-split vs. dithering vs. S/N trade-off.

Advanced Camera for Surveys Instrument Handbook for Cycle 23 > Chapter 7: Observing Techniques > 7.5 A Road Map for Optimizing Observations

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