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]
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. 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 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 buying 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. Some instruments use artificial rotation of the interferometer array instead of Earth rotation, such as in aperture masking interferometry.
History
See also: Synthetic aperture radar § History
Aperture synthesis imaging was first 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 kilometres. Aperture synthesis is also used by a type of radar system known as synthetic aperture radar, and even in optical telescopes.
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 synthesise the complete aperture (allowing image reconstruction by Fourier transform) to trying to synthesise the image from whatever data is available, using powerful but computationally expensive algorithms.
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)
References
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. Bibcode:1958MNRAS.118..276J. doi:10.1093/mnras/118.3.276.
Bernard F. Burke; Francis Graham-Smith (2010). An Introduction to Radio Astronomy. Cambridge University Press. ISBN 978-0-521-87808-1.
John D. Krauss (1966). "Chapter 6: Radio-Telescope Antennas". Radio Astronomy. New York, NY: McGraw Hill.
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. arXiv:1906.11239. Bibcode:2019ApJ...875L...2E. doi:10.3847/2041-8213/ab0c96.
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)
vte
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 Telescope (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 (Ukraine)
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) Arecibo Observatory (Puerto Rico, US) Green Bank Observatory (US) 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) Vermilion River Observatory (US)
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 Cyril Hazard Antony Hewish Sebastian von Hoerner 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
Aperture synthesis Cosmic microwave background radiation Interferometry Odd radio circle Pulsar timing array Radio propagation SETI
Wow! signal HD 164595 signal Solar radio emission
Hellenica World - Scientific Library
Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License
