Array Configurations

by Gustaaf Van Moorsel last modified Jun 23, 2017 by Emmanuel Momjian

Introduction

The VLA is reconfigurable and uses four principal array configurations, A through D. The A-configuration provides the longest baselines and thus the highest angular resolution for a given frequency, but yields very limited sensitivity to surface brightness. The D-configuration provides the shortest baselines, translating to a high surface brightness sensitivity at the cost of angular resolution. See the configuration schedule for details for each call for proposals and the highest angular resolution as function of frequency in the OSS. In general, as the baseline length expressed in wavelengths gets longer, the phase stability gets worse which impacts the observing strategy and observing overhead.

It is generally important to consider the following:

  • What angular resolution is required for the proposed science at the desired observing frequency?
  • For resolved sources, how does the desired angular resolution compare to the required surface brightness sensitivity?
  • For the array configuration that gives the desired angular resolution, how much of the flux density is actually in compact components, and will not be resolved out?

The Observational Status Summary section on Resolution, in conjunction with the Exposure Calculator, can help answer these questions.

Please note that low declination sources risk being subject to antenna shadowing at certain azimuths for the C and D configurations. These targets can still be observed. Observing at low declination implies, however, smaller windows of no-shadowing (one on either side of the north-south arm), which effectively makes the setup of the scheduling blocks harder or the observation less sensitive than expected.

Note on hybrid configurations: For very southern (and very northern) declinations, the VLA used to offer hybrid array configurations where the north arm was extended compared to the east and west arms. This provided a more circular synthesized beam at declinations south of −15 deg and north of 75 deg, at the cost of limited scheduling opportunities due to the short duration of the hybrids. Semester 2016A was the last semester to offer the hybrid configurations. See the following section for alternatives to the hybrids.

Alternatives to Hybrid Configurations

The approach required to substitute for the lack of hybrid VLA configurations depends on the science goal and observing mode of a proposal as described below. We assume projects that would have proposed to use the hybrid configurations are requesting them because they wish to observe sources at southern (<−15 deg) or northern (>75 deg) declinations.

Point Sources

Proposers should request the next largest principal configuration and ask for the same amount of observing time that would have been requested in a hybrid.

Extended Structure

Good surface brightness sensitivity is needed for projects aiming to image an extended structure. Assuming one is trying to match the surface brightness sensitivity of a hybrid configuration (rather than matching the surface brightness sensitivity to a particular science goal) we can use the density of visibilities, Nvis, as a function of uv-distance, as a measure of the sensitivity to different spatial scales. The sensitivity is then proportional to (Nvis)-1/2. The goal of matching the surface brightness sensitivity of a particular hybrid configuration is therefore one of matching the visibility density as a function of uv-distance through a combination of integration time in one or several principal configurations.

Figure 2.2.1 shows a graphical visualization useful for quantifying Nvis vs. uv-distance, for an example using ν=3GHz, δ=-25 deg, aiming to match the visibility density of the BnA hybrid for a snapshot observation. It demonstrates that to match the visibility density in the inner uv-plane of the BnA hybrid a combination of the same amount of integration time that would have been requested for the BnA hybrid (tint=tBnA) is needed in the A configuration, along with an additional 40% of the hybrid time (tint=0.4tBnA) in the B configuration (this combination is referred hereafter as A+0.4B). Alternatively, double the integration time in the A configuration (tint=2tBnA, hereafter 2A) can match the BnA visibility density at short uv spacings. A similar result is found for the CnB configuration.

Figure 2.2.2 shows how the additional time in the more compact configuration is a function of declination, and also the dependence on track length. A long (e.g., 5 or 6-hour) earth rotation synthesis is able to fill the uv-plane more effectively due to the decreased antenna shadowing when the target is away from the meridian compared with a snapshot or even a 2-hour synthesis centered on the meridian. BnA can be well reproduced by A+0.4B, and CnB can be reproduced by B+0.4C. For reproducing the visibility density of the DnC hybrid the shadowing on short baselines in the D configuration for δ<-25 deg is severe, and unless a long synthesis is possible it is more effective at these low declinations to double the hybrid time request and observe entirely in the C configuration.

Figure 2.2.3 presents the RMS noise as a function of the geometric mean FWHM of the synthesized beam derived for the same snapshot example in Figure 2.2.1 above, for a variety of uv-tapers and natural weighting. It demonstrates that A+0.4B matches well the surface brightness sensitivity of the BnA hybrid on spatial scales of at least 10 times the untapered, naturally weighted BnA synthesized beam of 2-arcsec, while 2A matches the RMS noise of the hybrid for spatial scales out to ~5 times the untapered, naturally weighted BnA synthesized beam.

In summary, then, the surface brightness sensitivity of a hybrid configuration can be reproduced by:
- either doubling the on-source integration time of the next largest principal configuration, or;
- combining 1.0 × (larger configuration) + 0.4 × (smaller configuration).
The choice of which to use depends on the science goal, declination, and observing mode, as described below. An exception is the DnC hybrid, for which shadowing at very low declinations makes the use of the D configuration very inefficient. To substitute for the DnC hybrid, proposers with targets at δ<-25 deg should always request double the amount of DnC observing time in C configuration.

  • Imaging extended structure with on-source integration times of around a minute or more:
  1. CnB/BnA substitute: proposers should combine 1.0 × (larger configuration) + 0.4 × (smaller configuration).
    • Note: if the field contains variable sources, but the science goal is imaging of extended structure, proposers should either request double the time in the next largest principal configuration (if this can be accommodated in a single scheduling block), or be prepared to model and subtract variable sources from individual datasets prior to combining, as needed.
  2. DnC substitute: proposers with targets at δ=-25 deg or higher should combine C+0.4D, as for CnB and BnA; proposers with targets at δ<-25 deg should request double the amount of DnC time but in the C configuration.
  • Imaging extended structure with very short on source integration times (e.g., large mosaics): very short scans can result in large slewing overheads, so to optimize observing efficiency proposers should request double the on-source time that would have been requested in the associated hybrid, for the next largest principal configuration.

For more details on how to optimize the science on the VLA without the hybrid configurations, we refer to the EVLA memo 193Also, proposers should direct any questions about which configuration they should use to the NRAO Helpdesk.