The VLA is a 27-element interferometric array which will produce images of the radio sky at a wide range of frequencies and resolutions. The basic data produced by the array 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 uses 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 commonly used for 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
through movement of its component antennas. There are four
basic arrangements, called configurations, whose scales vary by the
ratios 1 : 3.2 : 10 : 32 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
or
north of
.
Beginning in 1998, the standard C configuration was replaced by a slightly modified one, formerly known as CS, in which one antenna from the middle of the north arm (N10) is placed at N1 (at the center of the array) to give better short-spacing baseline coverage. See VLA Scientific Memos # 172 and 175, available from Reference 12 (see Section 6).
Traditionally, the VLA has completed one cycle through all four configurations in approximately a 16 month period. However, this cycle is likely to change beginning in 2007 in order to accommodate both the EVLA commissioning and an increase in time allocated to large observing programs. In particular, the configuration schedule is likely to change based on the delivery schedule for the EVLA correlator. One possible VLA configuration schedule for the upcoming years is outlined in Table 1, but prospective users should consult the web page http://www.vla.nrao.edu/genpub/configs/ or recent NRAO and AAS newsletters for up-to-date schedules and associated proposal deadlines. Refer to Section 4.1 for information on how to submit an observing proposal.
| Q1 | Q2 | Q3 | Q4 | |
| 2007 | D | D,A | A | A,B |
| 2008 | B | C | D,A | A |
| 2009 | A,D | D,C | C,B | B,A |
Note: The schedules for 2007 and 2008 are likely to change, based on pressure for large proposals, while that for 2009 depends heavily on the EVLA correlator delivery schedule.
Observing projects on the VLA 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 Section 3.17 for details.
The VLA can be broken into as many as five sub-arrays, each of which can observe a different object at a different band. This is especially useful for multi-band flux density monitoring observations, and for observing compact objects for which the VLA's full imaging capability and sensitivity are not required. However, important restrictions apply when multiple sub-arrays are used - refer to Section 3.9 for these restrictions.
All VLA antennas are permanently outfitted with receivers for seven
wavelength bands centered near
90, 20, 6, 3.6, 2.0, 1.3, and
0.7 cm. These bands are commonly referred to as P, L, C, X, U, K and
Q bands, respectively. However, the staging of the EVLA antenna retrofits
is such that the EVLA antennas have the old U band systems removed, and
they will not be replaced with new wideband systems until 2010 and later.
Thus, the maximum number of antennas with U band available will decrease
monotonically from approximately 20 at the beginning of 2007 to
approximately 14 at the beginning of 2008. In addition, a new receiving
system centered near 
1.0 cm (32 GHz) will begin to
be implemented on EVLA antennas in 2007, but a significant number
of antennas will not have this observing band until the end of 2007.
See Section 3.3 for more details about the availability
of new bands and enhanced frequency tunability for EVLA antennas.
The `4-band' system (better known as the 74 MHz system) is now available on all antennas, but is not permanently installed. The long dipoles needed to efficiently illuminate the antennas have been shown to reduce the gain and increase the system temperature at 20 cm by about 5%. Because of this, and the experimental nature of this frequency band, we will continue past practice of mounting the dipoles for limited periods of time during which 74 MHz observing is concentrated. At present, this means that, provided a sufficient number of project hours at 74 MHz have been approved, dipoles are mounted near the end of the A configuration for observations in the A, BnA and B configurations. Typically, there has not been sufficient proposal pressure to mount the dipoles in the more compact VLA configurations, although a special observing session for CnB and C configurations is being carried out in late 2006. The dipoles are removed after observations are completed in the B and/or C configurations.
The array can tune to two different frequencies from the same wavelength band provided the frequency difference does not exceed approximately 450 MHz. Right-hand circular (RCP) and left-hand circular (LCP) polarizations are received for both frequencies. Each of these four data streams is called an `IF' (for ``Intermediate Frequency'' channel). These four data streams are known in VLA-ese as IFs `A', `B', `C', and `D'. IFs A and B receive RCP, IFs C and D receive LCP. IFs A and C are always at the same frequency, as are IFs B and D. [But the (A,C) frequency is usually different than the (B,D) frequency.] Observations at more widely separated frequencies can only be made within the same run by time switching between the frequencies. This operation takes less than 30 seconds. The array also can observe simultaneously one frequency within L band and one within P band (known as LP), or one within 4-band and one within P band (known as 4P mode). These are the only currently supported modes in which frequencies within two different bands can be observed simultaneously.
Observations at seven different bandwidths (given by 
MHz,
with 
= 0, 1, ... 6) are possible1.
A 200 kHz bandwidth is also available in spectral line mode. Continuum
mode users wishing to use the 200 kHz bandwidth should first consult
VLA staff. Different bandwidths can be used for each of the two
separate frequencies. Wider bandwidths provide better sensitivity, but
also increase the chromatic aberration. Refer to Section
3.5.2 for details.
The VLA correlator has two basic modes, Continuum and Spectral Line. In Continuum mode, the correlator provides the four correlations (RR, RL, LR, LL) needed for full polarimetric imaging at both frequencies. This mode is particularly well suited to high sensitivity, narrow field-of-view projects. The Spectral Line mode is a spectrum-measuring mode principally intended for observing spectral lines. There are many options allowed in this mode. Besides its use for all spectral line projects, certain continuum projects which require extremely high dynamic range, or large field-of-view at high spatial resolution, will benefit from use of this mode. (These are sometimes referred to as multichannel continuum or pseudo-continuum projects.) This mode also is used to great advantage when RFI (Radio Frequency Interference) is expected within the bandpass. The Spectral Line modes are further described in Section 3.15.