Introduction

by Stephan W. Witz last modified Jun 22, 2017 by Emmanuel Momjian

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 D and A configurations for the observing period 2 March 2018 through 24 September 2018 (Semester 2018A), and should be used when preparing proposals for the 1 August 2017 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 the NRAO 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 page and the companion VLBA science web page.

The VLA is a large and complex modern instrument. Some familiarity with the principles and practices of its operation is necessary for its efficient use. 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. For more details, refer to the Visiting the DSOC and VLA page. The VLA is an extremely flexible instrument, and we are always interested in imaginative and innovative ways of using it.

 

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 produces images of the radio sky at a wide range of frequencies and resolutions. The VLA 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 (DSOC) 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. For details about antenna positions in the various configurations we refer to the stations position file (pdf version).

The VLA completes one cycle through all four configurations in an approximately 16 month period. Consult the Configuration Plans and Proposal Deadlines page or recent NRAO and AAS newsletters for current and up-to-date configuration schedules and associated proposal deadlines. Refer to the Guide to Proposing for the VLA for information on how to submit an observing proposal.

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 the section on Snapshots for more detail.

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 (L-band), 2–4 GHz (S-band), 4–8 GHz (C-band), 8–12 GHz (X-band), 12–18 GHz (Ku-band), 18–26.5 GHz (K-band), 26.5–40 GHz (Ka-band), and 40–50 GHz (Q-band). Additionally, all antennas of the VLA have receivers for lower frequencies, enabling observations at P-band (230–470 MHz). These low frequency receivers also work at 4-band (54–86 MHz), and new feeds have been deployed on a number of VLA antennas to observe at this frequency range.

The VLA correlator is both powerful and flexible. Details of the correlator configurations being offered for VLA science 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.