Fringe tracking methods for on-the-fly interferometry are considered. Several options are discussed, and it is concluded that those in which the desired phase function is discontinuous are undesirable. If the phase center is left at a fixed position on the sky while the antenna beams are scanned, then some loss of coherence occurs when the beams are away from the fixed position. The size of this loss is calculated, and it is found to be proportional to the total scan time and independent of observing frequency. For a given maximum loss and thus a maximum scan time, the number of beamwidths covered depends on the scanning speed.
For the worst-case ALMA parameters and 2% maximum loss, a linear scan can last about 6 sec and can cover 18 arcmin (limited by antenna speed) or 379 beamwidths (limited by correlator dumping rate), whichever is less.
View a PDF version of ALMA Memo #331.
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Last modified: November 03, 2000
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