Astronomical interferometer

An astronomical interferometer or telescope array is a set of separate telescopes, mirror segments, or radio telescope antennas that work together as a single telescope to provide higher resolution images of astronomical objects such as stars, nebulas and galaxies by means of interferometry. The advantage of this technique is that it can theoretically produce images with the angular resolution of a huge telescope with an aperture equal to the separation, called baseline, between the component telescopes. The main drawback is that it does not collect as much light as the complete instrument's mirror. Thus it is mainly useful for fine resolution of more luminous astronomical objects, such as close binary stars. Another drawback is that the maximum angular size of a detectable emission source is limited by the minimum gap between detectors in the collector array.^[1]

Interferometry is most widely used in

One simple layout of an astronomical interferometer is a parabolic arrangement of mirror pieces, giving a partially complete reflecting telescope but with a "sparse" or "dilute" aperture. In fact, the parabolic arrangement of the mirrors is not important, as long as the optical path lengths from the astronomical object to the beam combiner (focus) are the same as would be given by the complete mirror case. Instead, most existing arrays use a planar geometry, and Labeyrie's hypertelescope will use a spherical geometry.

History

One of the first uses of optical interferometry was applied by the

Astronomical interferometry is principally conducted using Michelson (and sometimes other type) interferometers.

Current projects will use interferometers to search for

High on the Chajnantor plateau in the Chilean Andes, the European Southern Observatory (ESO), together with its international partners, is building ALMA, which will gather radiation from some of the coldest objects in the Universe. ALMA will be a single telescope of a new design, composed initially of 66 high-precision antennas and operating at wavelengths of 0.3 to 9.6 mm. Its main 12-meter array will have fifty antennas, 12 metres in diameter, acting together as a single telescope – an interferometer. An additional compact array of four 12-metre and twelve 7-meter antennas will complement this. The antennas can be spread across the desert plateau over distances from 150 metres to 16 kilometres, which will give ALMA a powerful variable "zoom". It will be able to probe the Universe at millimetre and submillimetre wavelengths with unprecedented sensitivity and resolution, with a resolution up to ten times greater than the Hubble Space Telescope, and complementing images made with the VLT interferometer.

For details of individual instruments, see the list of astronomical interferometers at visible and infrared wavelengths.

A simple two-element optical interferometer. Light from two small lenses ) is combined using beam splitters at detectors 1, 2, 3 and 4. The elements creating a 1/4-wave delay in the light allow the phase and amplitude of the interference visibility to be measured, which give information about the shape of the light source.

A single large telescope with an aperture mask over it (labelled Mask), only allowing light through two small holes. The optical paths to detectors 1, 2, 3 and 4 are the same as in the left-hand figure, so this setup will give identical results. By moving the holes in the aperture mask and taking repeated measurements, images can be created using aperture synthesis which would have the same quality as would have been given by the right-hand telescope without the aperture mask. In an analogous way, the same image quality can be achieved by moving the small telescopes around in the left-hand figure — this is the basis of aperture synthesis, using widely separated small telescopes to simulate a giant telescope.

At radio wavelengths, interferometers such as the

Max Tegmark and Matias Zaldarriaga have proposed the Fast Fourier Transform Telescope which would rely on extensive computer power rather than standard lenses and mirrors.^[14] If Moore's law continues, such designs may become practical and cheap in a few years.

Progressing quantum computing might eventually allow more extensive use of interferometry, as newer proposals suggest.^[15]

See also

• Event Horizon Telescope (EHT) and Laser Interferometer Space Antenna (LISA)
• ExoLife Finder, a proposed hybrid interferometric telescope
• Hypertelescope
• Cambridge Optical Aperture Synthesis Telescope, an optical interferometer
• Navy Precision Optical Interferometer
  , a Michelson Optical Interferometer
• Radio astronomy § Radio interferometry
• Radio telescope § Radio interferometry

List

• 4C Array
• Akeno Giant Air Shower Array (AGASA)
• Allen Telescope Array (ATA), formerly known as the One Hectare Telescope (1hT)
• Antarctic Muon And Neutrino Detector Array (AMANDA)
• Atacama Large Millimeter Array (ALMA)
• Australia Telescope Compact Array
• CHARA array
• Cherenkov Telescope Array (CTA)
• Chicago Air Shower Array (CASA)
• Infrared Optical Telescope Array (IOTA)
• Interplanetary Scintillation Array (IPS array) also called the Pulsar Array
• LOFAR
  (LOw Frequency ARray)
• Modular Neutron Array (MoNA)
• Murchison Widefield Array (MWA)
• Northern Extended Millimeter Array (NOEMA)
• Nuclear Spectroscopic Telescope Array
  (NuSTAR)
• Square Kilometre Array (SKA)
• Submillimeter Array (SMA)
• Sunyaev-Zel'dovich Array
  (SZA)
• Telescope Array Project
• Very Large Array (VLA)
• Very Long Baseline Array (VLBA)
• Very Small Array

References