The radio interferometers is a sophisticated generation of radio telescopes, complex instruments that exploit the already known optical interferometry principles to greatly improve the resolving power, especially for large wavelengths.


Basic structures of radio interferometers with two antennas

 

Basic structures of radio interferometers with two antennas (product and sum).

 

Stanier (1950) was the first who showed as a radio interferometer was able to measure the Fourier components of the distribution of brightness of a radio source, while the radio astronomical developments of this technique are mainly due to M. Ryle (England, 1952) and Christiansen and Mills in Australia (1953), which extended two antennas radio interferometric techniques to those relating to multi-mobile elements structure, laying the foundation for the called aperture synthesis principle, formulated in detail by Ryle and Hewish in 1960.

The modern interferometric measurements using groups of radio telescopes distributed over very large (at the limit, the entire surface of the Earth, or on the earth and the space on board artificial satellites) that operate as the slits of a diffraction grating: as in this case, the resolving power of the interferometer depends on the total size of the instrument and can be made arbitrarily large. Today you can reach less decisive powers than a thousandth of a second of arc, which allow the study of celestial objects in considerable detail, such as, for example, the central parts of galaxies and quasars, where it originated the enormous amount of energy that is observed.

 

American NRAO Very Large Array

 

Suggestive aerial view (picture above) of the characteristic structure of the "single" greatest tool radio interferometric in the world: the VLA (Very Large Array - NRAO) in the American desert Socorro (New Mexico), consisting of 3 Y-shaped arms, each length of 35 km, with 9 parabolide antennas in each of a diameter of 25 meters (operating in the frequency range from 1 to 24 GHz). This impressive instrument is seen as a symbol of modern astronomy.

 

Institute for Radio Astronomy (IRA) in Bologna (Italy)

 

The Institute for Radio Astronomy (IRA) is one of the institutes of the National Research Council of Italy, now part of the National Institute for Astrophysics (INAF), and is the official body in charge of radio astronomy in Italy. The institute employs about 50 researchers: 14 astronomers, electrical engineers and computer 8 and 18 people that provide technical and administrative support. Work together, more than 10 researchers from the departments of Physics and Astronomy, University of Bologna.

 

The cross Northern Italian radio astronomy facility

 

Italian radio astronomy facility that includes the telescope "The Cross of the North" and the satellite dish antenna of 32 meters in diameter used for VLBI observations. The Institute manages three instruments: two satellite dishes made from the CNR and the telescope" Northern Cross" built by the University of Bologna in the early 60s.

  

The increase the resolving power

The general procedure of measuring cosmic flow density consists in pointing the telescope on the sky zone concerned by measuring the increase in the resulting signal with respect to a reference level. If the receiving apparatus has been calibrated, the deflection of the useful signal can be directly converted into equivalent temperature of antenna: known the antenna gain is possible to obtain the flux density of the source.

To reveal the flow of weak radio is essential to use an antenna system characterized by large effective area, while for distinguishing further structural details requires a high resolving power, reachable using instruments equipped with a sufficiently tight receive bundle.

This feature, which contributes to decrease the fluctuations of the background noise of the sky, is directly related to the physical dimensions of the antenna system, with considerable limits to the resolution achievable in the practice: the problem is solved using the interferometer systems that derive, in line principle, the Michelson instrument optical.

To push the resolving power beyond the limits achievable by a single antenna is used the composition of the signals coming from two or more antennas (each having relatively small size) placed at a distance large compared to their size and to the operating wavelength.

The radio interferometers and aperture synthesis in arrays of antennas have been developed for measuring purposes angular details of cosmic radiation, easily overcoming the resolution limits imposed by single antenna radio telescopes.