ALMA High-frequency Long-baseline Campaign in 2019: Band 9 and 10 In-band and Band-to-band Observations Using ALMA’s Longest Baselines

The Atacama Large Millimeter/submillimeter Array high-frequency long-baseline campaign in 2019 (HF-LBC-2019) was arranged to undertake band 9 (690 GHz) and 10 (850 GHz) observations using the longest 16 km baselines in order to explore calibration feasibility and imaging capabilities. Observations were arranged using close calibrators between 0° and 4° from the target point-source quasars (QSOs) to also explore subtle effects of calibrator separation angle. A total of 13 observations were made, five using standard in-band observations and eight using the band-to-band (B2B) observing mode, where phase solutions are transferred from a lower frequency band. At bands 9 and 10, image angular resolutions as high as 7 and 5 mas were achieved, respectively. Both in-band and B2B experiments were successful in imaging the target QSOs but with varying degrees of quality. Target image coherence varied between 0.14 and 0.79, driven by the calibrator separation angle and effectiveness of phase referencing despite observing in correct stability conditions. We conclude that the phase rms conditions and calibrator selection, specifically separation angle from the target, must carefully be considered prior to observing in order to minimize imaging defects. For bands 9 and 10, in order to achieve a coherence >0.7 such that the image structure and source flux can be regarded as suitably accurate, a 1° separated calibrator should be used while the phase rms over the phase switching cycle time should ideally be <30°.