An Overview of the VLA

by Stephan W. Witz last modified Jul 06, 2012 by Stephan Witz

The Karl G. Jansky Very Large Array (VLA) is a 27-element interferometric array, arranged along the arms of an upside-down "Y", which will produce images of the radio sky at a wide range of frequencies and resolutions. It is located at an elevation of 2100 meters on the Plains of San Agustin in southwestern New Mexico, and is managed from the Pete V. Domenici Science Operations Center (SOC) in Socorro, New Mexico.

The basic data produced by the VLA are the visibilities, or measures of the spatial coherence function, formed by correlation of signals from the array's elements.  The most common mode of operation will use these data, suitably calibrated, to form images of the radio sky as a function of sky position and frequency.   Another mode of observing (commonly called phased array) allows operation of the array as a single element through coherent summation of the individual antenna signals. This mode is most commonly used for Very Long Baseline Interferometry (VLBI) observing and for observations of rapidly varying objects, such as pulsars.

The VLA can vary its resolution over a range exceeding a factor of ∼ 50 through movement of its component antennas. There are four basic antenna arrangements, called configurations, whose scales vary by the ratios 1 : 3.28 : 10.8 : 35.5 from smallest to largest. These configurations are denoted D, C, B, and A, respectively. In addition, there are 3 "hybrid" configurations labelled DnC, CnB, and BnA, in which the North arm antennas are deployed in the next larger configuration than the SE and SW arm antennas. These hybrid configurations are especially well suited for observations of sources south of δ = −15° or north of δ = +75°, for which the foreshortening of the longer North arm results in a more circular point spread function.

The VLA completes one cycle through all four configurations in an approximately 16 month period.   The length of the cycle was modified during 2010 to accommodate commissioning of the new VLA correlator and the onset of early science with the Expanded VLA.  At the start of the 2013 observations, the VLA will again start the 16 month cycle.   The VLA configuration schedule for 2013 is presented in Table 3, but prospective users should consult the web page http://science.nrao.edu/facilities/evla/proposing/configpropdeadlines.shtml or recent NRAO and AAS newsletters for up-to-date schedules and associated proposal deadlines.  Refer to Obtaining Observing Time on the VLA for information on how to submit an observing proposal.

Table 3: VLA Configuration Schedule for 2013
Year 25 Jan - 22 Apr 26 Apr - 13 May24 May - 19 Aug 23 Aug - 9 Sep20 Sep - 16 Dec
2013 D
DnC C CnB B

Observing projects on the VLA will vary in duration from as short as 1/2 hour to as long as several weeks.  Most observing runs have durations of a few to 24 hours, with only one, or perhaps a few, target sources.  However, since the VLA is a two-dimensional array, images can be made with data durations of less than one minute.  This mode, commonly called snapshot mode, is well suited to surveys of relatively strong, isolated objects.  See Snapshots for details.

All VLA antennas are outfitted with eight receivers providing continuous frequency coverage from 1 to 50 GHz. These receivers will cover 1-2 GHz, 2-4 GHz, 4-8 GHz, 8-12 GHz, 12-18 GHz, 18-26.5 GHz, 26.5-40 GHz, and 40-50 GHz. These bands are commonly referred to as L, S, C, X, Ku, K, Ka, and Q bands, respectively.

The VLA correlator is both powerful and flexible.  Details of the correlator configurations being offered for VLA science in 2013  are described in Correlator Configurations.   It is important to realize that the VLA correlator is fundamentally a spectral line correlator.  The days of separate "continuum" and "spectral line" modes of the first generation VLA correlator are over, and all observations with the VLA are now "spectral line."    This has implications for how observations are set up and users who may be used to continuum observing with the VLA are strongly advised to consult Correlator Configurations.