VLA > Introduction

# Introduction

VLA capabilities February 2015 - September 2015

# 1. Purpose of Document, Older Versions of the OSS

This Observational Status Summary (OSS) summarizes the instrumental status of the Karl G. Jansky Very Large Array (VLA) for the B, BnA, and A configurations starting 6 February 2015, and should be used when preparing proposals for the 1 August 2014 deadline.  For capabilities prior to that date, we refer to our overview of all OSS versions available online.

The OSS is intended as a ready reference for those contemplating use of the VLA for their astronomical research. The information is in summary form - those requiring greater detail should use our Helpdesk, or refer to the manuals and documentation listed in Documentation. Most of the information contained here, and much more, is available through the VLA science web pages, and the companion document for the VLBA.

The VLA is a large and complex modern instrument. Some familiarity with the principles and practices of its operation is necessary for efficient use to be made of it.  Although the NRAO strives to make using the VLA as simple as possible, users must be aware that proper selection of observing mode and calibration technique is often crucial to the success of an observing program. Inexperienced and first-time users are encouraged to enlist the assistance of an experienced colleague or NRAO staff member for advice on, or direct participation in, an observing program. Refer to the Visiting the DSOC and VLA page for details. The VLA is an extremely flexible instrument, and we are always interested in imaginative and innovative ways of using it.

# 2. An Overview of the VLA

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.  For details about antenna positions in the various configurations we refer to the relevant postscript file.

The VLA completes one cycle through all four configurations in an approximately 16 month period.    The VLA configuration schedule for 2014/2015 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 the Guide to Proposing for the VLA for information on how to submit an observing proposal.

Table 3: VLA Configuration Schedule for 2014/2015

27 Jun - 17 Sep, 2014 19 Sep - 6 Oct, 201417 Oct - 5 Jan, 2015 9 Jan - 26 Jan, 20156 Feb - 11 May, 2015 15 May - 1 Jun, 201512 Jun - 21 Sep, 2015
Configuration: D DnC C CnB B BnA A

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 cover the frequency ranges of 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 and the bands are commonly referred to as L, S, C, X, Ku, K, Ka, and Q bands, respectively.  In addition, all antennas of the VLA now have even lower frequency receivers, covering 230-470 MHz (P-band).  And a small number are being outfitted with even lower frequency receivers, covering 54-86 MHz (4-band).

The VLA correlator is both powerful and flexible.  Details of the correlator configurations being offered for VLA science in 2015 are described in the WIDAR Section of this document.   It is important to realize that the VLA correlator is fundamentally a spectral line correlator and that even 'continuum' observations are done in a wide-band mode with many channels.

# 3. The Expanded Very Large Array (EVLA) Project

The Expanded VLA (EVLA) project modernized the VLA electronics (built in the 1970s and 1980s) in order to improve several key observational parameters by an order of magnitude or more.  Some of the details of the EVLA Project may be found at http://www.aoc.nrao.edu/evla/.   The EVLA project was funded jointly by the US National Science Foundation (NSF), the Canadian National Research Council, and the CONACyT funding agency in Mexico.   Total funding was approximately $94 million in Year 2006 dollars, including$59 million in new NSF funding, $16 million in redistributed effort from the NRAO Operations budget,$17 million for the correlator from Canada, and \$2 million from Mexico.

The EVLA project was completed on time and on budget at the end of 2012, 11 years after it began.  Its key observational goals were (1) complete frequency coverage from 1 to 50 GHz; (2) continuum sensitivity improvement by up to an order of magnitude (nearly two orders of magnitude in speed) by increasing the bandwidth from the VLA's 100 MHz per polarization to 8 GHz per polarization; and (3) implementation of a new correlator capable of processing the large bandwidth with a minimum of 16,384 spectral channels per baseline.  All goals were met.   A comparison of some of the EVLA performance parameters with those of the original VLA is provided in Table 1.

Note: The "Factor" gives the factor by which the EVLA parameter improves on the equivalent VLA parameter.

VLA to EVLA transition

The correlator that was the heart of the VLA for three decades was decommissioned on 11 January, 2010, and replaced with the new EVLA "WIDAR" correlator.  The VLA was shut down to outside users until March 2010, during which time hardware was transferred from the old correlator to the EVLA correlator and observing modes commissioned in preparation for EVLA early science. At the same time the direction of the configuration cycles also changed, from ABCDA to DCBAD, in order to facilitate the EVLA correlator commissioning and to limit initial EVLA data rates. The last VLA antenna was retrofitted to EVLA specifications in May 2010.

The first full configuration cycle of early science using the EVLA correlator saw up to 256 MHz of bandwidth offered to the general community, and 2 GHz bandwidth for observers willing to visit Socorro to help with EVLA commissioning.  By the end of 2011, at the start of the second configuration cycle, up to 2 GHz bandwidth was offered to the general community.  This increased to 8 GHz at the start of full operations in semester 2013A.