The MultiDrizzle Handbook

2.4 Observational Dither Strategies

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Here we provide, broad, general recommendations aimed at guiding observers toward different sampling strategies, depending upon the type of science that will be extracted from the data. Please note that these are guidelines only and in no way intended as solid rules. There will likely be science programs that do not fit exactly into any one of these categories, or that have different requirements.

2.4.1 No Dithering

Very Short Exposures: If each target is observed for less than a few minutes then the extra overhead for dithering can significantly impact the overall S/N, thus offsetting any advantages gained by dithering

Critical Photometric measurements: For high-precision time-dependent photometric monitoring, dithering may introduce additional complications due to intra-pixel sensitivity variations, thus some observers may prefer to obtain all the images at a single pointing location.

2.4.2 Simple Dithering

Even if sub-pixel dithering is not necessarily required, dithering each exposure by an integer pixel shift reduces the impact of hot pixels. To improve spatial sampling, 2- or 3-point sub-pixel dithering may be used, depending on how much overhead can be afforded. It is possible to do CR rejection with a single image at each dither point, although two or more images at each location will yield more robust rejection. For programs up to about one orbit per target, at least two to three exposures should be obtained to facilitate cosmic ray rejection. If one is interested in targets throughout the field, rather than one single star, cosmic ray removal will need to be more rigorous, and a larger number of exposures will be required. The instrument handbooks give expected cosmic ray rates for each of the imaging instruments.

2.4.3 "Full" Dithering

If improved spatial sampling is desired on programs of two or more orbits per target/filter combination, then a "full" 4-point dither is recommended (e.g., providing 1/2 pixel sub-sampling along both detector axes). Most of the sub-pixel information in an image is recovered by a four-point dither. However, for deep programs even larger numbers of dithers can be considered. Obtaining a four-point dither across the field of view limits the user to small dithers because of the distortion of many HST cameras. At the same time, the user may want to remove features such as the slit between the two chips on ACS with a large dither. The user may want to combine several sets of four-point dithers in this case. In addition, in cases where there are small objects with high signal-to-noise, image quality can be improved by using dithering patterns sampled finer than 4 points.

2.4.4 Dithering for Parallel Images

It is not always possible to obtain optimal dithers simultaneously for primary and parallel instruments due to the large separation and generally different pixel scales. Uniformly spaced dithers for the primary instrument generally yield non-uniform dithers for the parallel instrument. A specific exception are the Planetary Camera (PC) and Wide Field Cameras (WFCs) of WFPC2, where a pattern that produces a sub-pixel dither on both instruments has been developed (Section 6.4). However, in most cases, we recommend that users select their dither pattern in order to obtain the best possible data from their primary instrument.