Aperture synthesis

Aperture synthesisorsynthesis imagingis a type ofinterferometrythat mixes signals from a collection oftelescopesto produce images having the sameangular resolutionas an instrument the size of the entire collection.At each separation and orientation, the lobe-pattern of the interferometer produces an output which is one component of theFourier transformof the spatial distribution of the brightness of the observed object. The image (or "map" ) of the source is produced from these measurements.Astronomical interferometersare commonly used for high-resolutionoptical,infrared,submillimetreandradio astronomyobservations. For example, theEvent Horizon Telescopeproject derived the first image of a black hole using aperture synthesis.

Technical issues

Aperture synthesis is possible only if both theamplitudeand thephaseof 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. Seeastronomical interferometerfor 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 (n_b) for an array ofntelescopes is given byn_b=(n²−n)/2. (This is${\binom {n}{2}}$orⁿC₂). For example, theVery Large Arrayhas 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 paperA 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 inaperture masking interferometry.

History

See also:History of synthetic-aperture radar

The concept of aperture synthesis was first formulated in 1946 by Australianradio astronomersRuby Payne-ScottandJoseph Pawsey.Working fromDover HeightsinSydney,Payne-Scott carried out the earliestinterferometerobservations in radio astronomy on 26 January 1946 using anAustralian Armyradar as a radio telescope.

Aperture synthesis imaging was later developed at radio wavelengths byMartin Ryleand coworkers from theRadio Astronomy GroupatCambridge University.Martin Ryle andTony Hewishjointly received aNobel Prizefor this and other contributions to the development of radio interferometry.

The radio astronomy group in Cambridge went on to found theMullard Radio Astronomy Observatorynear Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as theTitan) 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 theOne-MileandRyletelescopes, respectively.

The technique was subsequently further developed invery-long-baseline interferometryto obtain baselines of thousands of kilometers and even inoptical telescopes.The termaperture synthesiscan also refer to a type ofradarsystem known assynthetic 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 afully sampledFourier 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 nameaperture 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-lineardeconvolutionalgorithms such as themaximum entropy method.The alternative namesynthesis imagingacknowledges 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.

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) andInverse synthetic-aperture radar(ISAR)
• Van Cittert–Zernike theorem#Aperture synthesis

External links

• Development of radio interferometry,fromAstronomical Optical Interferometry, A Literature Reviewby Bob Tubbs, Cambridge, 2002
• Cambridge Optical Aperture Synthesis Telescope
• APerture SYNthesis SIMulator (an interactive tool to learn the concepts of Aperture Synthesis)