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  • Primer
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Astronomical radio interferometry

Nature Reviews Methods Primers volume 3, Article number: 89 (2023) Cite this article

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Abstract

Radio interferometry and its application to arrays of element antennas enable sensitive studies of celestial objects with angular resolutions comparable with, or surpassing, optical imaging at wavelengths thousands of times shorter. The aperture synthesis technique offers the advantage of improving the angular resolution by effectively creating a telescope as large as the greatest separation between array elements. This Primer introduces radio interferometry systems that receive cosmic electromagnetic signals at submillimetre to metre wavelengths. First, the concept of aperture synthesis, the basic instrumental components and the calibration of data are described with an overview of currently operational astronomical arrays. The process of image synthesis and the factors that need to be considered in producing a radio astronomy image are described and common data formats and software applications for processing observation data are introduced. Various factors that limit the capabilities and/or optimization of arrays are outlined. Future plans for radio interferometry are presented to close the Primer.

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Fig. 1: (u, v) plane of a radio interferometric array.
Fig. 2: Simple astronomical radio interferometers.
Fig. 3: Synthesis imaging of an actual interferometric array of the Atacama Large Millimeter/submillimeter Array.
Fig. 4: Synthesis imaging with the CLEAN deconvolution (robust weighting with a robustness parameter of 1.0).
Fig. 5: Interferometric images for celestial objects.

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Data availability

Data described in this Primer are available from the Atacama Large Millimeter/submillimeter Array (ALMA) Science Archive. ALMA data of ADS/JAO.ALMA 2011.0.00011.SV (target source: VY CMa) were used for Figs. 3 and 4, and ADS/JAO.ALMA 2011.0.00015.SV (target source: HL Tau) for Fig. 5a.

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Acknowledgements

This Primer makes use of the following Atacama Large Millimeter/submillimeter Array (ALMA) data: ADS/JAO.ALMA 2011.0.00011.SV and ADS/JAO.ALMA 2011.0.00015.SV. ALMA is a partnership of the European Southern Observatory (ESO) (representing its member states), the National Science Foundation (NSF) (USA) and the National Institutes of Natural Sciences (NINS) (Japan), together with the National Research Council (NRC) (Canada), the Ministry of Science and Technology (MOST) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) (Taiwan), and the Korea Astronomy and Space Science Institute (KASI) (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the ESO, the AUI/National Radio Astronomy Observatory (NRAO) and the National Astronomical Observatory of Japan (NAOJ). The authors thank their colleagues for useful discussions.

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Authors and Affiliations

  1. Department of Science Operations, Joint ALMA Observatory, Santiago, Chile

    Yoshiharu Asaki & Belén Alcalde Pampliega

  2. National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Santiago, Chile

    Yoshiharu Asaki

  3. Astronomical Science Program, The Graduate University for Advanced Studies, SOKENDAI, Tokyo, Japan

    Yoshiharu Asaki & Satoru Iguchi

  4. European Southern Observatory, Santiago, Chile

    Belén Alcalde Pampliega

  5. CSIRO Space and Astronomy, Australia Telescope National Facility, Epping, New South Wales, Australia

    Philip G. Edwards

  6. National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Tokyo, Japan

    Satoru Iguchi

  7. National Radio Astronomy Observatory, Charlottesville, VA, USA

    Eric J. Murphy

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  1. Yoshiharu Asaki
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  2. Belén Alcalde Pampliega
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  3. Philip G. Edwards
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  4. Satoru Iguchi
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  5. Eric J. Murphy
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Contributions

Introduction (Y.A. and P.G.E.); Experimentation (Y.A., P.G.E., S.I. and E.J.M.); Results (Y.A. and P.G.E.); Applications (Y.A. and P.G.E.); Reproducibility and data deposition (B.A.P.); Limitations and optimizations (E.J.M.); Outlook (P.G.E. and E.J.M.); Overview of the Primer (all authors).

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Correspondence to Yoshiharu Asaki.

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Nature Reviews Methods Primers thanks Ciriaco Goddi, Violette Impellizzeri and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Related links

Astronomical Image Processing System (AIPS): http://www.aips.nrao.edu/

Common Astronomy Software Applications (CASA): https://casa.nrao.edu

Grenoble Image and Line Data Analysis System (GILDAS): https://www.iram.fr/IRAMFR/GILDAS/

PIMA: http://astrogeo.org/pima/

Glossary

Active galactic nuclei

(AGNs). The central regions of a distant extragalactic galaxy or quasar, which emit enormous energy in the frequency range from radio to γ-ray. It is believed that the energy output is powered by accretion towards a supermassive black hole at the centre of the AGN.

Atomic maser frequency standard

The accurate frequency generated from the atomic maser clock. There are several atomic species commercially used. The most popular is the hydrogen maser, which has the best frequency precision in time ranges shorter than 103 s. Caesium atomic clocks are also widely used because the precision for longer timescales is better than the other species. Rubidium frequency standards are not as precise, but can be sufficient for lower frequency observations.

Flux density

The energy flow rate, or spectral irradiance, of electromagnetic waves, measured as energy per unit time per unit area per unit frequency (W m−2 Hz−1). In radio astronomy, radio telescopes usually receive tiny power of signals per unit time, and a modified unit of jansky (1 Jy = 10−26 W m−2 Hz−1) is often used.

High electron-mobility transistor

A microwave device that makes use of a field-effect transistor incorporating a junction between two materials with different band gaps as the channel instead of a doped region. For a front-end device, it is often required to be cooled to suppress the receiver noise.

Maser

Microwave amplification by stimulated emission of radiation — the naturally occurring microwave analogue of a laser.

Radio window

The frequency region in metre to submillimetre wavelengths where Earth’s atmospheric opacity allows observing from the ground. This is greatly affected by the water vapour content in the Earth’s atmosphere.

Single-dish

A parabolic radio telescope antenna. A single-dish observation is a retronym for a non-interferometric observation.

Superconductor–insulator–superconductor

A device that converts millimetre/submillimetre waves to a lower frequency by mixing a local frequency signal using superconductor–insulator–superconductor tunnel junctions. Superconductor–insulator–superconductor devices must be cryogenically cooled to operate effectively.

Visibility

Output from an interferometer with amplitude and phase generated in the cross-correlation between two antennas. In the data reduction, the visibility is often treated as a complex number for simplifying calculations for the calibration and image synthesis using a two-dimensional (2D) Fourier transform.

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Asaki, Y., Alcalde Pampliega, B., Edwards, P.G. et al. Astronomical radio interferometry. Nat Rev Methods Primers 3, 89 (2023). https://doi.org/10.1038/s43586-023-00273-4

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