M.A. Holdaway [1], P.J. Napier [2], F. Owen [2]
August 1, 1996
Keywords: Atacama array, long baselines, phase stability, decorrelation, clustered array, phased array, configuration, correlator, bandwidth, sensitivity, field of view, image quality
One mode of collaboration between the LMSA and the MMA is to form a 10km array using all 90 antennas to gain the sensitivity required for such high resolution observations. This array has been called the ``Atacama Array''. One possible configuration of the 90 element Atacama Array is to arrange the antennas in clusters of 2-6 antennas, scattered in a ring-like shape. The signals from the antennas in a cluster would be phased up, and the phased cluster signals would be correlated, some by the LMSA correlator and some by the MMA correlator. The advantage of this scheme is that the number of correlations which must be performed is greatly reduced, permitting much wider bandwidth per visibility, and hence, continuum sensitivity which is a factor of 2-5 higher (depending upon the number of clusters) than using the two arrays separately. With 90 antennas, we can afford to trade Fourier plane coverage for bandwidth, but this trick would not work very well for the arrays individually. One antenna from each cluster could also be used for phase calibration. The disadvantages of this configuration include a reduced field of view, a reduced number of baselines resulting in poorer image quality, the possibility of atmospheric phase fluctuations across the cluster dephasing the signal, and greatly increased correlator complexity. The field of view problem can best be overcome by correlating all antennas (with a decrease in bandwidth), but most astronomical targets of the Atacama Array are very compact objects and would not be troubled by the field of view restrictions. The long track Fourier coverage of an array with 20-30 clusters is still quite good (comparable to the VLA, which produces excellent images), and we expect most astronomical targets of the Atacama Array will be sensitivity limited rather than limited by the quality of the Fourier plane coverage. The atmosphere at Chajnantor is good enough to permit the clusters to phase up at 950~GHz with less than 15% loss in sensitivity about 50% of the time. In fact, the phasing works much better than fast switching phase connection between the clusters. If radiometric phase correction worked better than fast switching, it would also work well enough to improve the phasing of the clusters.
In order to correlate signals from both the LMSA and the MMA in the guise of the Atacama Array, the two instruments must interface cleanly, which will require extensive design work. We feel that this early design work, drawing upon the expertise of both the NRO and the NRAO, should result in strengthening the designs of both instruments. The clustered array should be considered as a viable way to get the full collecting area of the Atacama Array while gaining, rather than losing, continuum bandwidth.
[1] NRAO/TUC
[2] NRAO/NM
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