2020ApJS..250...18M

The Atacama Large Millimeter/submillimeter Array (ALMA) obtains spatial resolutions of 15 to 5 mas at 275-950 GHz (0.87-0.32 mm) with 16 km baselines. Calibration at higher frequencies is challenging as ALMA sensitivity and quasar density decrease. The band-to-band (B2B) technique observes a detectable quasar at a lower frequency that is closer to the target, compared to one at the target high frequency. Calibration involves a nearly constant instrumental phase offset between the frequencies and the conversion of the temporal phases to the target frequency. The instrumental offsets are solved with a differential gain calibration (DGC) sequence, consisting of alternating low- and high-frequency scans of strong quasars. Here we compare B2B and in-band phase referencing for high frequencies (>289 GHz) using 2-15 km baselines and calibrator separation angles between ∼0.°68 and ∼11.°65. The analysis shows the following: (1) DGC for B2B produces a coherence loss <7% for DGC phase rms residuals <30°. (2) B2B images using close calibrators (<1.°67) are superior to in-band images using distant ones (>2.°42). (3) For more distant calibrators, B2B is preferred if it provides a calibrator ∼2° closer than the best in-band calibrator. (4) Decreasing image coherence and poorer image quality occur with increasing phase-calibrator separation angle because of uncertainties in the antenna positions and suboptimal phase referencing. (5) To achieve >70% coherence for long-baseline (16 km) band 7 (289 GHz) observations, calibrators should be within ∼4° of the target.