ALMA High-frequency Long-baseline Campaign in 2017: An Investigation of Phase-referencing Cycle Times and Effective Baseline Lengths Using Band-to-band and In-band Phase Calibration Techniques

Abstract The Atacama Large Millimeter/submillimeter Array can achieve unprecedented spatial resolutions of 15 to 5 mas with the longest baselines 16 km and observing at highest frequencies 275–950 GHz (1.09–0.30 mm). Two conditions are paramount for successful observations these frequencies: a phase calibrator in close proximity science target stable-enough atmosphere provide low residual rms post-phase-referencing. We investigate effect phase-referencing cycle times, in-band band-to-band (B2B) techniques, using distant calibrators, bands 7, 8, 9 >2000 m. find that (1) estimated from longer than effective baseline increases as function time . (2) expected coherence corresponds well achieved image coherence. (3) Faster times (<1 minute) improve calibration accuracy when calibrators (<2°) used. (4) For <2 minutes negative impact dominates error budget. also explore whether theoretical parameterization used estimate stability measurement, combined an empirical degradation separation, is suitable estimating images. latter proves be more accurate. Finally, we illustrate pragmatic approach establishing optimal strategy via dynamic on-calibrator scan lengths.