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The MultiDrizzle Handbook

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2.1 What is Dithering?


An increasingly popular technique in UV optical and IR imaging observations involves the use of dithering or spatially offsetting the telescope by shifts that are generally small relative to the detector size, thereby moving the target to a number of different locations on the detector. Two of the main strategies involve offsets by an integer number of pixels to facilitate the removal of bad pixels, and offsets by sub-integer pixels to improve spatial sampling of the point spread function (PSF). The latter application is particularly important in the case of HST, where the PSF is so small that it is significantly undersampled by the majority of the primary science instruments.

A third spatial offsetting technique involves the use of large shifts, comparable to the scale of the detector, to fully map areas of the sky that are several times larger than the detector area. This is generally referred to as mosaicing, and involves observational considerations and methods of data analysis that are beyond the scope of this document. However, the techniques we discuss in this document are essential to mosaicing with HST.


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