Radio interferometer arrays are used to achieve high angular resolution. As interferometers sample the Fourier components of the brightness distribution of radio sources, it is only necessary to sample well enough to systhesise an aperture much larger than any physical antenna structure than could be build in practise, and to reconstruct the image of the radio source as seen by the synthetic aperture. As interferometers not only have higher resolution than single dishes, but also perform a filtering of the spatial frequencies, they can eliminate confusing extended radiation from their maps and provide better position measurements for compact sources than single dishes.
The Very Large Array (VLA), near Socorro, New Mexico, is a synthesis instrument
which has twenty-seven 25m Cassegrain telescopes arranged along three 21 km
long arms of a 'Y' with nine telescopes in each. There are four standard
configurations, A (35 km maximum baseline), B (11 km), C (3.5 km) and D (1 km).
Sufficient distortion is introduced into the symmetry of the array by inclining
the N arm (north arm) of the 'Y' west of the true north-south line
in order to avoid that ellipses from different pairs of telescopes overlap in
the
plane (Fourier plane).
It is possible to map a source adequately from a single 8-hour observation. At 2 cm wavelength the angular resolution in the A configuration is 0.2 arcsec with a field of 2.9 arcmin. Both the resolution and the field scale roughly as the wavelength.