2022ApJS..259...10M

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 275-950 GHz (1.09-0.30 mm). Two conditions are paramount for successful observations at these frequencies: a phase calibrator in close proximity to the science target and a stable-enough atmosphere to provide a low residual phase rms post-phase-referencing. We investigate the effect of phase-referencing cycle times, for in-band and band-to-band (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(time). (2) The expected coherence estimated using the expected phase rms corresponds well to the achieved image coherence. (3) Faster phase-referencing 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 on-calibrator scan lengths.