Array Configurations

by Gustaaf Van Moorsel last modified Apr 02, 2018 by Lorant Sjouwerman

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. 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, as well as technical details regarding the above noted recommendations, we refer to the EVLA memo 193. Also, proposers should direct any questions about which configuration they should use to the NRAO Helpdesk. For an example on how to combine data from several array configuration, we refer to the VLA data combination guide.

Solar Observations

The Sun is a very extended source and has a complex brightness distribution that varies in time due to solar rotation and intrinsic source variation. For this reason, only a single configuration should be used to observe the Sun rather than combining different configurations. Furthermore, time variable phenomena such as flares require the use of the instantaneous, or snapshot, uv coverage provided by a configuration. The most useful configurations for observations of the Sun at frequencies above 1 GHz are the C and the D configurations. The A and the B configurations provide coverage that is too dilute and that over-resolves the Sun.