Science Highlight




Calibrator Catalogue

WARNING: Due to a software issue the calibrator source coordinates are not showing as columns in the Result Table tab. In order to access the coordinates there are three options: 1) switch to the "Hierarchical view" (bottom right button on the Result Table tab) 2) use the JSON download 3) access the Calibrator Catalogue through the OT to select and import source and coordinates.

A web-based user interface to the calibrator database is provided through the


The ALMA Calibrator Source Catalogue is a database of astronomical measurements of calibrator sources, mostly bright quasars in the mm and sub-mm regime. It contains over 50000 ALMA measurements of over 3000 sources (1 January 2021). The most important properties are flux density with uncertainty at a given frequency and angular structure information or limits, and polarization measurements for some calibrators. External information had been provided via 'seed' catalogues, such as from VLA ( Calibrator Manual ), SMA[1] ,   ATCA , CRATES , and coordinates from VLBI , and we acknowledge the institutions and individuals who contributed to this effort. Note that the absolute flux density scale does not include the model uncertainty of the solar system object used as primary amplitude calibrator, and in some cases it can exceed the stated flux density error in the catalogue. Also, the observation and reduction procedures changed in mid-2012, so the measurements before this date may state flux density errors that underestimate the true uncertainty. 

A main use of the database is to allow the selection of bandpass and phase calibrators for science observations. It also contains a set of 35 quasars, grid sources evenly distributed over the sky, which are monitored regularly enough to provide secondary amplitude calibration in addition to solar system objects (primarily based on the Uranus model, ALMA memo 594). Calibrator sources are selected automatically via on-line queries during the execution of the observation, in execeptional cases also manually during phase 2 preparation. In each case suitable criteria regarding visibility, flux density and proximity to the science targets have to be specified. 

Calibrator observations, outside of science observations, are carried out in special observing modes, partially integrated into science operations with data reduction automated via the ALMA Pipeline. Access for users is provided via a web-based user interface through the ALMA Regional Centers.

The Query Form allows to specify various search criteria regarding position, frequency, flux density and time of the calibrator measurements. Equatorial coordinates in ICRS can be specified for the positional search. If no time constraint is given, the latest measurements per day, band and source will be returned. If a time constraint is given, all measurements will be shown with the results table sorted hierarchically by sources. Frequencies can be selected by ALMA band and/or with an additional frequency range. A given source name will first be resolved within the ALMA database, but if no match is found, the coordinates will be resolved via Sesame with a suggested search radius for a cross-check with the ALMA catalogue. The ALMA standard name is defined as the truncated 4-digit RA and DEC coordinates following the IAU standard. 

At least one catalogue, within the Calibrator Source Catalogue, should be selected for the measurement search. The default catalogue is ‘ALMA’ containing all measurements obtained as part of the on-going monitoring of potential calibrator sources[2] for the observation of science projects, such as grid monitoring of flux calibrators and cone searches for phase calibrator selection, reduced with special manual scripts and Pipeline recipes. The ‘ALMA_QA2’ catalogue contains high S/N Pipeline reduction results on phase calibrators from science data. The ‘VLBI’ catalogue contains the original measurements from ICRF with the coordinates adopted for ALMA use, except for rare cases where different coordinates have been found in ALMA observations. The ‘ALMA_TP_off’ catalogue contains clean off-positions for single dish observing that have been checked for Galactic molecular line emission in observations and been adopted by ALMA. 

The "Type" attributed to source can also be specified, which is by default "Point source" for all ALMA calibrators. Searches for Total Power OFF positions require the Type to be selected as "TP off position" (together with selecting the catalogue "ALMA_TP_off"). Other useful Types to select are for example "Flux calibrator" to be used as such in observations, "Grid source" which are frequently monitored, or "Line source" which are not used as bandpass calibrators due to extensive line contamination. 

The Result Table can be formatted by adding or removing columns, and each column can be sorted. There is an option to download the result in a file (csv or json format). Structure information, expressed as the acceptable uv range, is available for sources where relevant for ranges of ALMA baseline.  Uvmin and uvmax columns specify information about the structure of the source. Uvmin gives a detected extent of the source on baselines in klambda (or zero for a limit) and uvmax gives a detection of internal structure of the source resolved on baselines in klambda (or a limit). Coordinates in ICRS and coordinate errors are taken from the VLBI  catalogues[3] (rfc_2017b). Flux density results are normally averaged over all spectral windows in a band, apart from band 3, where two results are given (for both sidebands, i.e. a lower and higher frequency separated by 12 GHz). Stated flux density uncertainties do not in all cases fully account for uncertainties in the planetary models used for the primary amplitude calibration. 

The Result Plot page displays a positional plot showing all sources as dots colored by bands, which can be specified, and their size representing the latest flux density. A flux density history plot opens when clicking on a source. Both plots can be zoomed, and further source and measurement information is provided in tooltips.

A VO compatible interface for flux estimation can be queried via the URL https://almascience.org/sc/flux?DATE=15-Oct-2019&FREQUENCY=233.0E+09&NAME=J0006-0623 giving an XML table as output. For well-sampled sources, such as grid sources, the flux density and spectral index is fitted in time and frequency, and for infrequently monitored sources the nearest suitable measurements in time are used for the estimate. 

A detailed description of the Flux Service can be found here.

Questions and comments on the use of the interface and the calibrator catalogue are welcome via the ALMA help-desk.

[1] Updated 2016, Mark Gurwell, private communication

[2] See ALMA Technical Handbook, Section 10.2.6 (Source Catalogue) and E. Fomalont, T. van Kempen, R. Kneissl, et al., The Calibration of ALMA using Radio Sources, 2014, Msngr, 155 19F.

[3] L. Petrov, C. Phillips, A. Bertarini, T. Murphy, E. M. Sadler, The LBA Calibrator Survey of southern compact extragalactic radio sources - LCS1, 2011, Mon. Not. Roy. Astron. Soc., 414(3), 2528–2539; L. Petrov, A. de Witt, E. M. Sadler, C. Phillips, S. Horiuchi, The Second LBA Calibrator Survey of southern compact extragalactic radio sources - LCS2, 2019, Mon. Not. Roy. Astron. Soc., 485(1), 88-101.