The MultiDrizzle Handbook

2.3 Costs and Drawbacks of Dithering

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While dithering provides substantial benefits, there are a number of trade-offs that must be understood and considered when deciding whether or not to obtain dithered data. These are described more fully later in the document but are summarized here:

• Obtaining the final combined data product will require special reductions and thus more work on the part of the observer.
• Some extra spacecraft overhead time will be incurred, and observers will need to judge if this is significant by actually running tests on various scenarios using the RPS2 scheduling software.
• If longer exposures are broken into shorter exposures to obtain more dithers, the user may suffer an increase in read noise. The increased volume of data will fill the data buffer faster, possibly complicating the observing.
• If the primary science goal is to measure differential changes over time, as in time-series photometry, then dithering can in some cases complicate the resulting analysis due to intra-pixel sensitivity variations in the detectors. This was primarily a concern with NIC3, and should not be an issue for WFC3 which has not shown the same intra-pixel sensitivity as NICMOS during its ground testing.
• Elimination of cosmic rays may be slightly compromised, especially if one has only a few sub-pixel offsets with just one image at each location.

For most observing programs on HST the potential drawbacks to dithering are outweighed by the scientific benefits. However, in specific instances it might be possible that the above drawbacks are deemed too severe, as in programs with very few available orbits. If you have questions about how your particular program would be affected by dithering, and you do not find the answers in this document, please feel free to get in touch with help@stsci.edu or your Contact Scientist if one has been assigned.