From Wikipedia:
The
Very Large Array (
VLA) is a
radioastronomy observatory located on the
Plains of San Augustin, some fifty miles (80 km) west of
Socorro,
New Mexico,
USA. The VLA stands at an altitude of 6970 ft (2124 m) above sea level. It is a component of the
National Radio Astronomy Observatory (NRAO).
The observatory consists of 27 independent antennas, each of which has a dish diameter of 25 meters (82 feet) and weighs 209
metric tonnes (230
Short tons).
The antennas are arrayed along the three arms of a Y-shape (each of which measures 21 km/13 miles long). Using the railroad tracks that follow each of these arms – and that, at one point, intersect with
U.S.Route 60 at a level crossing – and a specially designed lifting locomotive, the antennas can be physically relocated to a number of prepared positions,allowing
aperture synthesis interferometry with a maximum baseline of 36 km (22.3 miles): in essence, the array acts as a single antenna with that diameter. The smallest angular resolution that can be reached is about 0.05
arcseconds at a
wavelength of 7 mm.
There are four commonly used configurations, designated A (the largest) through D (the tightest, when all the dishes are within 600 m of the center point). The observatory normally cycles through all the various possible configurations (including several hybrids) every 16 months; the antennas are moved every three to four months.
Moves to smaller configurations are done in two stages, first shortening the east and west arms and later shortening the north arm. This allows for a short period of improved imaging of extremely northerly or southerly sources.
