The ALMA Calibrator Database I: Measurements taken during the commissioning phase of ALMA
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ALMA Memo # 599
The ALMA Calibrator Database I: Measurements taken during the commissioning phase of ALMA
T. A. van Kempen, R. Kneissl, N. Marcelino, E.B. Fomalont, D. Barkats, S.A. Corder, R. Lucas, A.B. Peck, and R. Hills
2014-12-02
Interferometric ALMA observations rely upon accurate and trustworthy calibration of the instrumental and atmospheric phase and amplitude variations in order to produce high quality scientific data. Short observations (scans) of radio sources at the appropriate cadences are included in observing programs in order to determine these variations with time and frequency. Quasars are among the best radio source candidates because their emission is bright, lack spectral features and are point-like; typically they measure less than 0.01" in size. Since quasar flux densities vary both in time and frequency, specialized ALMA observations are needed to find the quasars that are sufficiently bright between 90 and 900 GHz, the ALMA frequency range. Once found, the appropriate quasar flux densities must be monitored with periodic ALMA observations to track. Hence, in 2011, the ALMA calibrator database was initiated during the commissioning and science verification stage of ALMA. This memo describes the database structure and contents, the observational and data reduction strategy, and discusses many properties of the radio sources observed to this date. During the commissioning phase, two major observing programs were started: First, a wide survey of about 600 quasars was carried out with the aim to obtain a sufficiently large sample of sources to be used as phase reference sources. Second, a selection of about 45 bright and relatively stable quasars, called grid sources , were monitored regularly. The sample was distributed relatively equally over the sky. These are candidates for bandpass calibrations, but have since also evolved into secondary amplitude calibrator candidates. Both programs have continued into the operational phase of ALMA.
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