ALMA High-frequency Long-baseline Campaign in 2017: A Comparison of the Band-to-band and In-band Phase Calibration Techniques and Phase-calibrator Separation Angles

Abstract

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.

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

“…In some cases, observations were terminated early but still have two DGC source visits. These so-called DGC source blocks are ∼10 minutes long and consist of 10 lowfrequency 18 s duration scans interleaved with nine highfrequency scans of 32 s duration (see Asaki et al 2020a;Maud et al 2020). For the B2B mode, an independent bandpass source was not observed for the low-frequency band as the DGC source low-frequency scans are suitable for this purpose.…”

“…The band 9 tests covered three ranges, 13:00-16:00, 16:00-20:00, and 20:00-02:00, as it was easier to find suitable QSO combinations. Paired in-band and B2B observations were arranged to run one after the other to ensure the best continuation of weather and phase stability conditions such that a fair comparison of the two modes could be made, although not all of our experiments were continuous due to time constraints (see Maud et al 2020). The QSOs selected as targets and calibrators were known suitable sources based on our HF-LBC-2017 studies and allowed us to use phasecalibrator-to-target separations angles between 0°.…”

“…The main deviation from standard PI calibration for the inband data was that we used a single flux calibrator for amplitude correction because of the difficulty in finding a secondary amplitude gain calibrator at high-frequency bands (Maud et al 2020). Typically, for standard data reduction, the phase calibrator is used as a secondary gain calibrator where amplitude gains are solved temporally, but bootstrapped from the main flux calibrator.…”

“…The first ALMA long-baseline observations in bands 3, 4, and 6 were highlighted by ALMA Partnership et al (2015a, 2015c, 2015d and since 2015, ALMA has been observing with long baselines at these bands (and shortly thereafter band 5). Following from the aforementioned investigations, further studies were conducted to explore the long-baseline capability at higher frequencies (Asaki et al 2020a(Asaki et al , 2020bMaud et al 2020Maud et al , 2022, which played a key role in Cycle 7 being the first to offer band 7 (273-373 GHz) long-baseline observations where resolutions of 15-20 mas were achievable. Only very recently for ALMA Cycle 9 (starting 2022 October), as a result of the high-frequency long-baseline campaign 2019 (HF-LBC-2019) data we publish here, ALMA opened band 8 observations with 16 km baselines, while slightly extending the array configurations for band 9 and band 10 to use maximal baselines of 13.9 km and 8.5 km, respectively.…”

“…This phase calibration method is called band-to-band (B2B; see Asaki et al 2016Asaki et al , 2020a for details) and is the ALMA version of similar frequency-phase-transfer techniques used at other facilities (e.g., Dodson & Rioja 2009;Pérez et al 2010;Rioja & Dodson 2011;Rioja et al 2015). Maud et al (2020) specifically examined any differences between the standard inband phase calibration and the B2B technique and the effect of phase calibrator separation angles as part of the ALMA highfrequency long-baseline campaign in 2017 (HF-LBC-2017). They reported that images of the targeted point sources were degraded in terms of image coherence (which is often referred to as the fractional peak flux recovered; e.g., Dodson & Rioja 2009;Rioja & Dodson 2011;Rioja et al 2015; and is the peak flux density of an image after phase referencing as compared to the peak flux density of a self-calibrated ideal image) losses and in more extreme cases by noticeable image defects, away from pointlike emission, when increasingly distant phase calibrators were used.…”