Aperture synthesis

Short description: Mixing signals from many telescopes to produce images with high angular resolution

Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection.[1][2][3] At each separation and orientation, the lobe-pattern of the interferometer produces an output which is one component of the Fourier transform of the spatial distribution of the brightness of the observed object. The image (or "map") of the source is produced from these measurements. Astronomical interferometers are commonly used for high-resolution optical, infrared, submillimetre and radio astronomy observations. For example, the Event Horizon Telescope project derived the first image of a black hole using aperture synthesis.[4]

## Contents

* 1 Technical issues
* 2 History
* 3 See also
* 4 References
* 5 External links

## Technical issues

Aperture synthesis is possible only if both the amplitude and the phase of the incoming signal are measured by each telescope. For radio frequencies, this is possible by electronics, while for optical frequencies, the electromagnetic field cannot be measured directly and correlated in software, but must be propagated by sensitive optics and interfered optically. Accurate optical delay and atmospheric wavefront aberration correction are required, a very demanding technology that became possible only in the 1990s. This is why imaging with aperture synthesis has been used successfully in radio astronomy since the 1950s and in optical/infrared astronomy only since the turn of the millennium. See astronomical interferometer for more information.

In order to produce a high quality image, a large number of different separations between different telescopes is required (the projected separation between any two telescopes as seen from the radio source is called a baseline) – as many different baselines as possible are required in order to get a good quality image. The number of baselines (nb) for an array of n telescopes is given by nb=(n2 − n)/2. (This is (n2) or nC2). For example, the Very Large Array has 27 telescopes giving 351 independent baselines at once, and can give high quality images.

Most aperture synthesis interferometers use the rotation of the Earth to increase the number of baseline orientations included in an observation. In this example with the Earth represented as a grey sphere, the baseline between telescope A and telescope B changes angle with time as viewed from the radio source as the Earth rotates. Taking data at different times thus provides measurements with different telescope separations.

In contrast to radio arrays, the largest optical arrays currently have only 6 telescopes, giving poorer image quality from the 15 baselines between the telescopes.

Most radio frequency aperture synthesis interferometers use the rotation of the Earth to increase the number of different baselines included in an observation (see diagram on right). Taking data at different times provides measurements with different telescope separations and angles without the need for additional telescopes or moving the telescopes manually, as the rotation of the Earth moves the telescopes to new baselines.

The use of Earth rotation was discussed in detail in the 1950 paper A preliminary survey of the radio stars in the Northern Hemisphere.[5] Some instruments use artificial rotation of the interferometer array instead of Earth rotation, such as in aperture masking interferometry.

## History

The concept of aperture synthesis was first formulated in 1946 by Australian radio astronomers Ruby Payne-Scott and Joseph Pawsey. Working from Dover Heights in Sydney, Payne-Scott carried out the earliest interferometer observations in radio astronomy on 26 January 1946 using an Australian Army radar as a radio telescope.[6]

Aperture synthesis imaging was later developed at radio wavelengths by Martin Ryle and coworkers from the Radio Astronomy Group at Cambridge University. Martin Ryle and Tony Hewish jointly received a Nobel Prize for this and other contributions to the development of radio interferometry.

The radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the Titan) became capable of handling the computationally intensive Fourier transform inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5 km' effective aperture using the One-Mile and Ryle telescopes, respectively.

The technique was subsequently further developed in very-long-baseline interferometry to obtain baselines of thousands of kilometers and even in optical telescopes. The term aperture synthesis can also refer to a type of radar system known as synthetic aperture radar, but this is technically unrelated to the radio astronomy method and developed independently.

Originally it was thought necessary to make measurements at essentially every baseline length and orientation out to some maximum: such a fully sampled Fourier transform formally contains the information exactly equivalent to the image from a conventional telescope with an aperture diameter equal to the maximum baseline, hence the name aperture synthesis.

It was rapidly discovered that in many cases, useful images could be made with a relatively sparse and irregular set of baselines, especially with the help of non-linear deconvolution algorithms such as the maximum entropy method. The alternative name synthesis imaging acknowledges the shift in emphasis from trying to synthesize the complete aperture (allowing image reconstruction by Fourier transform) to trying to synthesize the image from whatever data is available, using powerful but computationally expensive algorithms.

To avoid this computational bottleneck, telescopes such as the DSA-2000 use a large number (in this case 2000) randomly distributed antennas to nearly fully sample the uv plane. This gives a point spread function that allows much simpler reconstruction, so images can be computed as the data comes off the telescope. This is referred to as a radio camera.

Note that aperture synthesis is technically and historically independent of synthetic aperture radar, which is a Doppler technique, originally developed in the early 1950s by Carl A. Wiley. The operating principles are unrelated.[7]

## See also

* Beamforming
* Interferometric synthetic-aperture radar (IfSAR or InSAR)
* Light field
* Optical heterodyne detection (SAHD)
* Synthetic-aperture magnetometry
* Synthetic-aperture sonar
* Synthetic-aperture radar (SAR) and Inverse synthetic-aperture radar (ISAR)
* Van Cittert–Zernike theorem

## References

1. ↑ R. C. Jennison (1958). "A Phase Sensitive Interferometer Technique for the Measurement of the Fourier Transforms of Spatial Brightness Distributions of Small Angular Extent". Monthly Notices of the Royal Astronomical Society 119 (3): 276–284. doi:10.1093/mnras/118.3.276. Bibcode: 1958MNRAS.118..276J.
2. ↑ Bernard F. Burke; Francis Graham-Smith (2010). An Introduction to Radio Astronomy. Cambridge University Press. ISBN 978-0-521-87808-1. https://books.google.com/books?id=4dI6isxCmEcC.
3. ↑ John D. Krauss (1966). "Chapter 6: Radio-Telescope Antennas". Radio Astronomy. New York, NY: McGraw Hill.
4. ↑ The Event Horizon Telescope Collaboration (April 10, 2019). "First M87 Event Horizon Telescope Results. II. Array and Instrumentation". The Astrophysical Journal Letters 87 (1): L2. doi:10.3847/2041-8213/ab0c96. Bibcode: 2019ApJ...875L...2E.
5. ↑ A preliminary survey of the radio stars in the Northern Hemisphere
6. ↑ "National Radio Astronomy Observatory". https://www.nrao.edu/news/newsletters/enews/enews_2_11/ruby.shtml.
7. ↑ "Lockheed Martin: Synthetic Aperture Radar: “Round the Clock Reconnaissance”". https://www.lockheedmartin.com/en-us/news/features/history/sar.html.

## External links

* Development of radio interferometry, from Astronomical Optical Interferometry, A Literature Review by Bob Tubbs, Cambridge, 2002
* Cambridge Optical Aperture Synthesis Telescope
* APerture SYNthesis SIMulator (an interactive tool to learn the concepts of Aperture Synthesis)

* v
* t
* e

Radio astronomy

Concepts|

* Units (watt and jansky)
* Radio telescope (Radio window)
* Astronomical interferometer (History)
* Very Long Baseline Interferometry (VLBI)
* Astronomical radio source

Radio telescopes
(List)|

| Individual
telescopes|

* 500 meter Aperture Spherical Telescope (FAST, China)
* Arecibo Observatory (Puerto Rico, US)
* Caltech Submillimeter Observatory (CSO, US)
* Effelsberg Telescope (Germany)
* Galenki RT-70 (Russia)
* Green Bank Telescope (West Virginia, US)
* Large Millimeter Telescope (Mexico)
* Lovell Telescope (UK)
* Ooty Telescope (India)
* Qitai Radio Telescope (China)
* RATAN-600 Radio Telescope (Russia)
* Sardinia Radio Telescope (Italy)
* Suffa RT-70 (Uzbekistan)
* Usuda Telescope (Japan)
* UTR-2 decameter radio telescope (Ukraine)
* Yevpatoria RT-70 (Russia)

Southern Hemisphere
HartRAO (South Africa)
Parkes Observatory (Australia)
Warkworth Radio Astronomical Observatory (NZ)

|

Interferometers|

* Allen Telescope Array (ATA, California, US)
* Atacama Large Millimeter Array (ALMA, Chile)
* Australia Telescope Compact Array (ATCA, Australia)
* Australian Square Kilometre Array Pathfinder (ASKAP, Australia)
* Canadian Hydrogen Intensity Mapping Experiment (CHIME, Canada)
* Combined Array for Research in Millimeter-wave Astronomy (CARMA, California, US)
* European VLBI Network (Europe)
* Event Horizon Telescope (EHT)
* Giant Metrewave Radio Telescope (GMRT, India)
* Green Bank Interferometer (GBI, West Virginia, US)
* Korean VLBI Network (KVN, South Korea)
* Large Latin American Millimeter Array (LLAMA, Argentina/Brazil)
* Long Wavelength Array (LWA, New Mexico, US)
* Low-Frequency Array (LOFAR, Netherlands)
* MeerKAT (South Africa)
* Molonglo Observatory Synthesis Telescope (MOST, Australia)
* Multi-Element Radio Linked Interferometer Network (MERLIN, UK)
* Murchison Widefield Array (MWA, Australia)
* Northern Cross Radio Telescope (Italy)
* Northern Extended Millimeter Array (France)
* One-Mile Telescope (UK)
* Primeval Structure Telescope (PaST, China)
* Square Kilometre Array (SKA, Australia, South Africa)
* Submillimeter Array (SMA, US)
* Very Large Array (VLA, New Mexico, US)
* Very Long Baseline Array (VLBA, US)
* Westerbork Synthesis Radio Telescope (WSRT, Netherlands)

Space-based|

* HALCA (Japan)
* Spektr-R (Russia)

Observatories|

* Algonquin Radio Observatory (Canada)
* Haystack Observatory (US)
* Jodrell Bank Observatory (UK)
* Mullard Radio Astronomy Observatory (UK)
* National Radio Astronomy Observatory (US)
* Nançay Radio Observatory (France)
* Onsala Space Observatory (Sweden)
* Pushchino Radio Astronomy Observatory (PRAO ASC LPI, Russia)
* Special Astrophysical Observatory of the Russian Academy of Science (SAORAS, Russia)
* Warkworth Radio Astronomical Observatory

Multi-use|

* DRAO (Canada)
* ESA New Norcia (Australia)
* PARL (Canada)

People|

* Elizabeth Alexander
* John G. Bolton
* Edward George Bowen
* Ronald Bracewell
* Jocelyn Bell Burnell
* Arthur Covington
* Nan Dieter-Conklin
* Frank Drake
* Antony Hewish
* Karl Guthe Jansky
* Kenneth Kellermann
* Frank J. Kerr
* John D. Kraus
* Bernard Lovell
* Jan Oort
* Joseph Lade Pawsey
* Ruby Payne-Scott
* Arno Penzias
* Grote Reber
* Martin Ryle
* Govind Swarup
* Gart Westerhout
* Paul Wild
* Robert Wilson

Related articles|

* Radio propagation
* Interferometry
* Aperture synthesis
* SETI (Wow! signal, Radio signal from HD 164595)
* Pulsar timing array
* Cosmic microwave background radiation

* Submillimetre astronomy
* Infrared astronomy
* Optical astronomy
* High-energy astronomy
* Gravitational-wave astronomy

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