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46.9 Plate Distortion

Calibrated WF/PC-1 data from the pipeline are left with a residual geometric distortion; across a chip, from corner to corner, there is roughly a 1 pixel distortion in positional accuracy. This distortion can be removed, using the wmosaic task, which joins the four images into a single, correctly aligned and geometrically rectified, image; the residual distortion is about 0.1 pixels in both cameras. Note however, that wmosaic uses linear interpolation so the photometric accuracy of the image it produces is reduced. Thus, we recommend using the unrectified images for accurate photometry wherever possible. The task metric can be used to determine accurate RA and Dec for sources (correcting for the geometric distortion) directly from the uncorrected single chip images which make up the groups of the .c0h file. See "Basic Astrometry" on page 3-9 for more details.



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