Interferometer (radio astronomy)

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An interferometer from radio telescopes is used to achieve a high angular resolution with many smaller individual systems . This leads to sharper images of radio sources that are close together or of compact quasars and is also of great importance in the case of spatially extended radiation sources.

The distance between the outermost segments of a perforated parabolic mirror determines the resolution.
Reconstructed image of a radio source with two receiving antennas
Reconstructed intensity distribution of a radiation source with many linearly arranged antennas. The maxima can be localized more precisely.
Reconstructed image of a radiation source with a cross-shaped or L-shaped arrangement of the receiving antennas

Basics

The radio telescopes used in radio astronomy differ from the reflector telescopes used in optical astronomy only because of their considerably larger wavelength . However, identical physical laws apply when it comes to the resolution of distant objects. The Dawes criterion provides the estimate (Δα in radians )

which is also confirmed theoretically . A reflecting telescope with a diameter of 10 m can theoretically achieve a value of Δα ≈ 0.011  arc seconds with green light , in practice 0.2 arc seconds must be calculated. The very large array with a diameter of 36 km can achieve a best angular resolution of 0.04 arc seconds at a wavelength of 7 mm, measured as 0.05 arc seconds. This is exceeded by the Very Long Baseline Array with a greatest distance D of 8611 km and Δα ≈ 0.001 arc seconds - 500 times better than with optical devices!

Separation into individual systems

The Dawes criterion does not state that all photons that are “captured” within the opening diameter D actually have to be evaluated. The resolving power is not impaired if only the edge rays incident at distance D are evaluated. However, this means accepting massive problems with ambiguity.

As shown in the top picture, the surface of a parabolic mirror can also be perforated for reasons of wind load. It is very difficult to ensure the parabolic shape of a very large mirror (see Effelsberg radio telescope ) when moving and tilting. Fixed mirrors like those at the Arecibo Observatory limit the possibilities of observation too much, which is why one goes over to arrangements of separate individual antennas . The blue segments in the top image are deformed into individual paraboloids and built at a mutual distance with a separate receiver in focus . The image is reconstructed in the computer from their contributions, whereby several problems have to be solved:

  • There is no common focus of the overall system, but several whose signals must be merged. The connecting lines should be of identical length.
  • The "flat bending" of the parabolic shape (see picture) and its geometric arrangement generate very large phase differences in the vibrations received, which have to be compensated. Mastering the phase shifts is key to the process.
  • The antennas are firmly connected to the earth, which rotates relative to the stars. This creates variable phase changes that have to be controlled by phase shifters as with the phased array antenna . In both cases, the "viewing direction" is determined by setting the phases.
  • In the case of interferometers, the parabolic antennas also have to be swiveled.
  • Multiple images caused by interference patterns must be suppressed.

Receiving antennas and arrangement

Comparison of the resolution of an optical image of the Hubble Space Telescope (top right) with the synthetic image of two interferometers of different baseline lengths
  • If two antennas lying next to each other in east-west direction, whose distance exceeds the receiving wavelength, receive the waves from a distant, point-shaped radiation source, the reconstruction provides a striped pattern as in the double-slit experiment . This is ambiguous and very fuzzy in the east-west direction and does not allow any position information in the north-south direction.
  • If the number of equidistant antennas in a linear arrangement is increased, the arrangement corresponds to an optical grating which increases the spatial resolution in the east-west direction, but is still ambiguous. The uselessness in north-south direction is not improved.
  • If the east-west arrangement is supplemented by a similar north-south arrangement of further antennas, the reconstructed image is still not clear, but the location information is narrowed, as can be seen in the third image. The antenna “arrays” do not have to cross each other geometrically, they may even have a certain distance.

The reason for these ambiguities is the same spacing between the antennas. A clear localization of objects requires as many different distances ( baseline ) between the antennas as possible, which must also be oriented differently. Only then does a different interference pattern arise for each combination of two antenna signals. If you add a sufficient number, a single accumulation point remains per object and the image quality becomes acceptable. With n telescopes you can

select different baselines (see the combinatorics below ). On the Very Large Array with its 27 antennas, a maximum of 351 baselines can be set independently of one another. The star-shaped rail system also enables different orientations.

Realized solutions

Development of the phase difference with an oblique "viewing direction".

Technically, radio interferometry works by superimposing the signals from two or more radio telescopes . The overlay can be done electrically, if direct cable connections between the telescopes are practical, or it is simulated on computers . With the Very Long Baseline Array , the signals received are saved together with precise time stamps and transmitted via the Internet. With special interferometry methods such as the VLBI, one looks for the best correlation between two signals iteratively by shifting them against each other in small steps.

The superimposed signal is evaluated with the help of mathematical methods of Fourier analysis . The result is a map of the observed area, which ideally has the same resolution as from a radio telescope with a diameter that corresponds to the distance between the individual antennas of the interferometer.

Examples of radio interferometers:

  • The Atacama Large Millimeter / submillimeter Array (ALMA for short) consists of 66 antennas and is the largest radio telescope in the world. It is located at an altitude of about 5000 m in the Atacama Desert in the northern Chilean Andes.
  • The Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO) near Socorro, New Mexico, USA . It consists of 27 individual antennas that can be moved on three tracks at an angle of 120 degrees to each other.
  • Very Long Baseline Interferometry (VLBI): Pairs of radio telescopes that are very far apart, such as As the at Effelsberg standing Effelsberg radio telescope of the Max Planck Institute for Radio Astronomy in Bonn and the radio telescope at Green Bank Observatory in West Virginia, USA, measured at the same time the same astronomical object with the same wavelength. At both radio telescopes, the measurement signal isprovided with time markingsusing atomic clocks , which are so precise that the two signals can later be combined with one another in the computer.
  • Special case of interferometry with an antenna : Under certain conditions it is possible to obtain a superposition of the radio waves from a source by mirroring . In analog television , such a reflection, e.g. B. on a building , to a "ghost image" shifted to the right. Is a radio telescope z. B. on a cliff by the sea, astronomical objects that are flat above the sea can be observed both as a sea reflection and as an original image and the superimposition of the signals can be evaluated.

In any case, the interferometry only leads to a result if the path difference of the different signals changes, because in the Fourier transformation the strength of the signal is evaluated depending on the path difference. Since interferometers in radio astronomy are mostly based on the rotation of the earth, the path length changes in the east-west direction, so that the resolution is mainly improved in this direction.

See also

literature

  • Rudolf Wohlleben, Helmut Mattes, Thomas Krichbaum: Interferometry in radioastronomy and radar techniques. Kluwer, Dordrecht 1991, ISBN 0-7923-0464-0

Web links