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We describe the calibration and imaging heuristics developed and deployed in the ALMA interferometric data processing pipeline, as of ALMA Cycle 9. The pipeline software framework is written in Python, with each data reduction stage layered on top of tasks and toolkit functions provided by the Common Astronomy Software Applications package. This framework supports a variety of tasks for observatory operations, including science data quality assurance, observing mode commissioning, and user reprocessing. It supports ALMA and VLA interferometric data along with ALMA and NRO45m single dish data, via different stages and heuristics. In addition to producing calibration tables, calibrated measurement sets, and cleaned images, the pipeline creates a WebLog which serves as the primary interface for verifying the data quality assurance by the observatory and for examining the contents of the data by the user. Following the adoption of the pipeline by ALMA Operations in 2014, the heuristics have been refined through annual development cycles, culminating in a new pipeline release aligned with the start of each ALMA Cycle of observations. Initial development focused on basic calibration and flagging heuristics (Cycles 2-3), followed by imaging heuristics (Cycles 4-5), refinement of the flagging and imaging heuristics with parallel processing (Cycles 6-7), addition of the moment difference analysis to improve continuum channel identification (2020 release), addition of a spectral renormalization stage (Cycle 8), and improvement in low SNR calibration heuristics (Cycle 9). In the two most recent Cycles, 97% of ALMA datasets were calibrated and imaged with the pipeline, ensuring long-term automated reproducibility. We conclude with a brief description of plans for future additions, including self-calibration, multi-configuration imaging, and calibration and imaging of full polarization data., Comment: Accepted for publication by Publications of the Astronomical Society of the Pacific, 65 pages, 20 figures, 10 tables, 2 appendices. Small corrections and additions from the proof stage have been applied

A diverse array of science goals require accurate flux calibration of observations with the Atacama Large Millimeter/Submillimeter array (ALMA), however, this goal remains challenging due to the stochastic time-variability of the ``grid'' quasars ALMA uses for calibration. In this work, we use 343.5 GHz (Band 7) ALMA Atacama Compact Array observations of four bright and stable young stellar objects over 7 epochs to independently assess the accuracy of the ALMA flux calibration and to refine the relative calibration across epochs. The use of these four extra calibrators allow us to achieve an unprecedented relative ALMA calibration accuracy of $\sim 3\%$. On the other hand, when the observatory calibrator catalog is not up-to-date, the Band 7 data calibrated by the ALMA pipeline may have a flux calibration poorer than the nominal 10\%, which can be exacerbated by weather-related phase decorrelation when self-calibration of the science target is either not possible or not attempted. We also uncover a relative flux calibration uncertainty between spectral windows of 0.8\%, implying that measuring spectral indices within a single ALMA band is likely highly uncertain. We thus recommend various methods for science goals requiring high flux accuracy and robust calibration, in particular, the observation of additional calibrators combined with a relative calibration strategy, and observation of solar system objects for high absolute accuracy.

In this lecture, we describe a number of advanced gain calibration techniques. In particular, self-calibration is an important tool in interferometric imaging at all wavelengths. It allows the observer to determine and remove residual phase and amplitude errors that remain in the data after normal calibration while simultaneously producing a more accurate and more sensitive image of the target source. We describe the basic technique of self-calibration and attempt to dispel some common misconceptions. We proceed to give a range of useful tips, and provide continuum, spectral line, and mosaic self-calibration examples using ALMA data. We also discuss fast switching and radiometric phase correction along with advanced phase transfer techniques that can become advantageous or even essential at high frequency where the density of sufficiently bright phase calibrators becomes sparse., Comment: 31 pages, 13 figures, 56 references; to appear in the Proceedings of the 2014 Synthesis Imaging Workshop held in Socorro, NM on May 13-20, 2014

In a radio interferometer, the determination of geometrical antenna positions relies on accurate calibration of the dry and wet delay of the atmosphere above each antenna. For the Atacama Large Millimeter/Submillimeter Array (ALMA), which has baseline lengths up to 16 kilometers, the geography of the site forces the height above mean sea level of the more distant antenna pads to be significantly lower than the central array. Thus, both the ground level meteorological values and the total water column can be quite different between antennas in the extended configurations. During 2015, a network of six additional weather stations was installed to monitor pressure, temperature, relative humidity and wind velocity, in order to test whether inclusion of these parameters could improve the repeatability of antenna position determinations in these configurations. We present an analysis of the data obtained during the ALMA Long Baseline Campaign of Oct. through Nov. 2015. The repeatability of antenna position measurements typically degrades as a function of antenna distance. Also, the scatter is more than three times worse in the vertical direction than in the local tangent plane, suggesting that a systematic effect is limiting the measurements. So far we have explored correcting the delay model for deviations from hydrostatic equilibrium in the measured air pressure and separating the partial pressure of water from the total pressure using water vapor radiometer (WVR) data. Correcting for these combined effects still does not provide a good match to the residual position errors in the vertical direction. One hypothesis is that the current model of water vapor may be too simple to fully remove the day-to-day variations in the wet delay. We describe possible avenues of improvement, including measuring and applying more accurate values of the sky coupling efficiency of the WVRs., Comment: 19 pages, 14 figures, 1 table; presented at SPIE Astronomical Telescopes + Instrumentation 2016, held in Edinburgh, UK on 26 June - 1 July 2016

The primary antenna elements and receivers are two of the most important components in a synthesis telescope. Together they are responsible for locking onto an astronomical source in both direction and frequency, capturing its radiation, and converting it into signals suitable for digitization and correlation. The properties and performance of antennas and receivers can affect the quality of the synthesized images in a number of fundamental ways. In this lecture, their most relevant design and performance parameters are reviewed, with emphasis on the current ALMA and VLA systems. We discuss in detail the shape of the primary beam and the components of aperture efficiency, and we present the basics of holography, pointing, and servo control. On receivers, we outline the use of amplifiers and mixers both in the cryogenic front-end and in the room temperature back-end signal path. The essential properties of precision local oscillators (LOs), phase lock loops (PLLs), and LO modulation techniques are also described. We provide a demonstration of the method used during ALMA observations to measure the receiver and system sensitivity as a function of frequency. Finally, we offer a brief derivation and numerical simulation of the radiometer equation., Comment: 31 pages, 13 figures, 153 references; to appear in the Proceedings of the 2014 Synthesis Imaging Workshop held in Socorro, NM on May 13-20, 2014

We present a derivation of the radiometer equation based on the original references and fundamental statistical concepts. We then perform numerical simulations of white noise to illustrate the radiometer equation in action. Finally, we generate 1/f and 1/f^2 noise, demonstrate that it is non-stationary, and use it to simulate the effect of gain fluctuations on radiometer performance., Comment: 4 pages, 9 figures, to appear in the Proceedings of the 26th International Symposium on Space Terahertz Technology, available at https://www.cfa.harvard.edu/events/2015/isstt2015/proceedings/

We have constructed a five station 12 GHz atmospheric phase interferometer (API) for the Submillimeter Array (SMA) located near the summit of Mauna Kea, Hawaii. Operating at the base of unoccupied SMA antenna pads, each station employs a commercial low noise mixing block coupled to a 0.7 m off-axis satellite dish which receives a broadband, white noise-like signal from a geostationary satellite. The signals are processed by an analog correlator to produce the phase delays between all pairs of stations with projected baselines ranging from 33 to 261 m. Each baseline's amplitude and phase is measured continuously at a rate of 8 kHz, processed, averaged and output at 10 Hz. Further signal processing and data reduction is accomplished with a Linux computer, including the removal of the diurnal motion of the target satellite. The placement of the stations below ground level with an environmental shield combined with the use of low temperature coefficient, buried fiber optic cables provides excellent system stability. The sensitivity in terms of rms path length is 1.3 microns which corresponds to phase deviations of about 1 degree of phase at the highest operating frequency of the SMA. The two primary data products are: (1) standard deviations of observed phase over various time scales, and (2) phase structure functions. These real-time statistical data measured by the API in the direction of the satellite provide an estimate of the phase front distortion experienced by the concurrent SMA astronomical observations. The API data also play an important role, along with the local opacity measurements and weather predictions, in helping to plan the scheduling of science observations on the telescope., Comment: 23 pages, 10 figures, accepted for publication in the Journal of Astronomical Instrumentation

We describe the successful design, implementation, and operation of a 12 GHz holography system installed on the Robert C. Byrd Green Bank Telescope (GBT). We have used a geostationary satellite beacon to construct high-resolution holographic images of the telescope mirror surface irregularities. These images have allowed us to infer and apply improved position offsets for the 2209 actuators which control the active surface of the primary mirror, thereby achieving a dramatic reduction in the total surface error (from 390 microns to ~240 microns, rms). We have also performed manual adjustments of the corner offsets for a few panels. The expected improvement in the radiometric aperture efficiency has been rigorously modeled and confirmed at 43 GHz and 90 GHz. The improvement in the telescope beam pattern has also been measured at 11.7 GHz with greater than 60 dB of dynamic range. Symmetric features in the beam pattern have emerged which are consistent with a repetitive pattern in the aperture due to systematic panel distortions. By computing average images for each tier of panels from the holography images, we confirm that the magnitude and direction of the panel distortions, in response to the combination of gravity and thermal gradients, are in general agreement with finite-element model predictions. The holography system is now fully integrated into the GBT control system, and by enabling the telescope staff to monitor the health of the individual actuators, it continues to be an essential tool to support high-frequency observations., Comment: Accepted for publication in PASP. Contains 28 pages with 2 tables and 9 figures (several at reduced quality). The full resolution version is available at http://wwwlocal.gb.nrao.edu/ptcs/papers/Hunter2011/gbtholo.ps.gz (34MB gzip file unpacks to 134MB postscript)

Wind-induced pointing errors are a serious concern for large-aperture high-frequency radio telescopes. In this paper, we describe the implementation of an optical quadrant detector instrument that can detect and provide a correction signal for wind-induced pointing errors on the 100m diameter Green Bank Telescope (GBT). The instrument was calibrated using a combination of astronomical measurements and metrology. We find that the main wind-induced pointing errors on time scales of minutes are caused by the feedarm being blown along the direction of the wind vector. We also find that wind-induced structural excitation is virtually non-existent. We have implemented offline software to apply pointing corrections to the data from imaging instruments such as the MUSTANG 3.3 mm bolometer array, which can recover ~70% of sensitivity lost due to wind-induced pointing errors. We have also performed preliminary tests that show great promise for correcting these pointing errors in real-time using the telescope's subreflector servo system in combination with the quadrant detector signal., Comment: 17 pages, 11 figures; accepted for publication in PASP

The digital phase lock loop described in this paper has been in use on the Submillimeter Array (SMA) front-end receivers for over a decade and has been a key element in obtaining all of the high quality images that have been published from this telescope over the years. The technical achievements enabled by these devices include the first phase closure observations in the 690 GHz band, the first attempts at band-to-band phase transfer at submillimeter wavelengths, and the first successful demonstration of interferometry using a fully photonic millimeter-wave local oscillator. Copies of these devices are also in place at the Caltech Submillimeter Observatory and the James Clerk Maxwell Telescope in support of the eSMA project and submillimeter VLBI experiments. Additional units of this design were used by the Princeton Millimeter Interferometer and the Microwave Anisotropy Telescope. In total, over three dozen units have been constructed and used in astronomical studies. In this paper, we briefly describe the background theory, design, performance, and calibration steps, and provide useful testing and repair information., Comment: 19 pages, 6 figures. This is mostly a superset of SMA technical memo #143 but with a new introduction and additional citations