ALMA Highfrequency Longbaseline Campaign in 2017: An Investigation of Phasereferencing Cycle Times and Effective Baseline Lengths Using Bandtoband and Inband Phase Calibration Techniques
Abstract
The Atacama Large Millimeter/submillimeter Array can achieve unprecedented spatial resolutions of 15 to 5 mas with the longest baselines of 16 km and observing at the highest frequencies of 275950 GHz (1.090.30 mm). Two conditions are paramount for successful observations at these frequencies: a phase calibrator in close proximity to the science target and a stableenough atmosphere to provide a low residual phase rms postphasereferencing. We investigate the effect of phasereferencing cycle times, for inband and bandtoband (B2B) observing techniques, using close and distant phase calibrators, at bands 7, 8, and 9 for baselines >2000 m. We find that (1) the phase rms estimated from baselines longer than the effective baseline increases as a function of $\sqrt{{\rm{time}}}$ . (2) The expected coherence estimated using the expected phase rms corresponds well to the achieved image coherence. (3) Faster phasereferencing cycle times (<1 minute) improve phase calibration accuracy and image coherence when close phase calibrators (<2°) are used. (4) For cycle times <2 minutes the negative impact of distant phase calibrators dominates the phase error budget. We also explore whether a theoretical parameterization of the effective baseline used to estimate the phase rms and a phase stability measurement, combined with an empirical parameterization of coherence degradation as a function of calibrator separation, is suitable in estimating the expected coherence of target images. The latter proves to be more accurate. Finally, we illustrate a pragmatic approach in establishing an optimal observing strategy via dynamic cycle times and oncalibrator scan lengths.
 Publication:

The Astrophysical Journal Supplement Series
 Pub Date:
 March 2022
 DOI:
 10.3847/15384365/ac3b57
 Bibcode:
 2022ApJS..259...10M
 Keywords:

 113;
 932;
 1647;
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