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Paper: Lessons from the VLA Long Wavelength Sky Survey (VLSS)
Volume: 345, From Clark Lake to the Long Wavelength Array: Bill Erickson's Radio Science
Page: 337
Authors: Cotton, W.D.
Abstract: The VLA Low Frequency Sky Survey (VLSS) which uses the VLA at 74 MHz has provided significant lessons for future use of the VLA and other low frequency instruments. The two principle lessons are 1) don't depend on celestial observations to calibrate your instrument and 2) understand how to deal with the ionosphere. At low frequencies and long baselines, the size of a phase coherent region in the ionosphere, the “isoplanatic patch”, is smaller than the field of view of typical elements used to form the array. Except for the case of a single, confined source dominating the field, observations in this regime require spatially variant phase calibration. For the VLA at 74 MHz for much of the time, a simple low order approximation is sufficient. For larger arrays with lower frequencies, such as the LWA or LOFAR, higher order approximations will be needed; how to determine these corrections is not currently understood. Poorly behaved station beams will also be a calibration and imaging problem for future low frequency arrays.
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