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Paper: Bandwidth and Time-Average Smearing
Volume: 180, Synthesis Imaging in Radio Astronomy II
Page: 371
Authors: Bridle, A. H.; Schwab, F. R.
Abstract: This is the first of three lectures to deal with problems in imaging wide fields-of-view. Its goal is to quantify the first two effects described in Lecture 17---bandwidth smearing and time-average smearing. Both effects cause the synthesized image to be distorted in ways that cannot adequately be described (except locally) as a convolution of the true sky brightness distribution with a spatially invariant point source response function. Rather, the degree of smearing is a function of angular distance from the delay-tracking center (for bandwidth smearing) or the phase-tracking center (for time-average smearing). The effects therefore persist after simple (position-independent) deconvolution with methods like `CLEAN' or MEM. Since these distortions cannot be remedied by calibration (or self-calibration), it is important to devise synthesis observing strategies that hold the distortions down to acceptable levels. We wish now to characterize the two effects mathematically and to justify the approximations embodied in the practical formulae used elsewhere.
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