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DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Appendix A: Plate Scales and Polynomial Distortions > A.1 Introduction

A.1 Introduction
Apart from uncertainties in telescope pointing, astrometric accuracy is also affected by varying plate scale across detectors as a result of geometric distortion produced by the telescope and instrument optics.
For instance, distortion across the WF chips in WFPC2 amounts to ~2% at the chip corners. If the telescope was commanded to move exactly 2.5", that shift would correspond to a shift of 25 pixels at the detector center where the plate scale is 0.1 arcseconds/pixel. At the corners, however, where the plate scale is ~2% smaller, 0.098 arcseconds/pixel, that shift would have spanned over 25.5 pixels. In other words, objects near the edge of the chip would be subsampled by half a pixel, while those near the center would not be subsampled at all. Therefore, for any given shift in units of pixels, there is a continuously varying amount of subsampling across the image. This illustrates how detector distortion leads to non-uniform pixel subsampling when dithering.
The size of the additional offset introduced by geometric distortion towards the edges of the chip scales linearly with the size of the commanded telescope offset. For example, for WFPC2/WFC, a dither of five pixels at the center (instead of 25 pixels) would produce a much less severe error of only 0.1 pixels in additional shift at the edge of the chip. As such, the sampling would remain nearly constant across the detector. This is a reason for why large dithers are discouraged; although the Drizzle software is capable of dealing with changing pixel shifts and different degrees of subsampling across the image, the scientific interpretation of the data could still be impacted because not all the objects are equally subsampled. Image analysis would be greatly simplified if small dithers were used, thereby keeping such sampling differences relatively small across the chip (e.g., < 0.1 pixel). However, for the ACS, the two chips that compose the WFC are separated by a gap of about 2.5 arcseconds (~50 WFC pixels). Therefore, many ACS dither strategies involve the use of offsets sufficiently large to allow the detectors to cover this gap; the consequence is a varying degrees of subsampling across the detector.

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