ALMA Configurations

MDC Array Configuration Group Reports (1996)

ALMA Configuration Working Group

Configuration Design Concepts

The current ALMA configuration concept is based on a set of discrete or continuous arrays ranging from 150m to 10000m. The compact array is a filled array to optimize surface brightness sensitivity, and the other arrays are designed to optimize Fourier plane coverage and resolution. We are currently refining the ALMA configuration concept. The result of this process may result in a set of arrays that are very similar to or somewhat different from the current configuration concept. Here is a list of the issues involved in the array configuration design:

• Several Configurations. The 64 ALMA antennas will all be movable so the array can be reconfigured. Different configurations allow us to tailor the array's brightness sensitivity and resolution to the problem at hand.
• Compact Configuration. To optimize the brightness sensitivity for large scale structure, the most compact configuration will have the antennas as close together as possible. With 64 antennas of 12m, and a 1.3D minimum separation, the compact configuration will be a filled circle 150m in diameter, leading to a centrally condensed Fourier plane distribution. Full resolution is about 1.4 arcsec at 1mm.
• Largest Configuration. Formerly, it was assumed that 3000m would be the size of the largest configuration, resulting in a full resolution of about 0.06 arcsec at 1mm. However, the 1995 Science Workshop Group Reports indicate that there is much interest in higher resolution. We are investigating configurations as large as 10km.
• Fourier Plane Distribution. The desired Fourier plane distribution is essential to the configuration design. We are investigating the properties of various distributions:
• A ring-like antenna configuration will yield a nearly uniform Fourier plane distribution. Such an array will have the most long baselines of any physical antenna distribution, and will have optimal imaging of features which are barely resolved. The synthesized beam will be similar to a J1 Bessel function with large close-in sidelobes, and the wings of the main lobe will be much narrower than a Gaussian fit to the peak. To what extent are these features desirable or undesirable? To what extent can they be altered by optimizing the placement of the antennas about a ring-like configuration?
• A random antenna configuration will yield a linearly decreasing Fourier plane distribution which results in a more nearly Gaussian beam with lower side lobes. When the array is tapered to a lower resolution, less sensitivity is lost than in the case of a ring-like array with a uniform Fourier distribution.
• More centrally condensed Fourier plane distributions, such as that of the VLA, are not being considered because too much sensitivity is lost in reweighting to get a nearly Gaussian beam as the array has very little sensitivity on the long baselines.
• How many intermediate configurations? We can achieve better brightness sensitivity by tapering the (u,v) coverage, or by reconfiguring the antennas into a smaller array. In order to optimize the surface brightness for all resolutions, we are driven to many different arrays. The jump between the brightness sensitivity of the largest array tapered down to the next largest array's resolution, and that array's brightness sensitivity, depends upon both the Fourier plane distribution and the jump in the size of the arrays. Hence, by specifying what jump in brightness sensitivity we can live with, the Fourier plane distribution which we desire, and the minimum and maximum array sizes, we can determine how many configurations we need. The number of configurations will need to compromise with financial constraints, as each configuration requires road, cables, trenching, and antenna pads. A set of double ring arrays with a resolution jump of about factor 2 and a continuously reconfigurable spiral zoom array are two main competing designs being considered. Ring-like configurations can only share a subset of stations and small stretches of road and cable. Spiral zoom configurations with centrally condensed (u,v) distributions may be able to share more antenna pads, but may require more roads and cable. We are investigating these issues now.
• Some Short Baselines for Every Configuration! Unlike the VLA, the ALMA will have some short baselines for even the largest configurations so that we will often not require multi-configuration observations.
• Multiple Configuration Observing. While we must permit single configuration observing by designing short baselines into each configuration, observers will often observe in multiple configurations (for instance, detection with a compact array and detailed mapping in a larger array). We need to explore the observing situations in which observers will have multi-configuration data, and how this should affect the configuration designs. This may feed back into the number of intermediate configurations and the desired Fourier plane distribution.
• Low Elevation Observing. Partial reconfiguration between adjacent arrays should allow us to build north-south stretched arrays which have a more circular beam for low elevation (ie, far southern or northern declinations) observing.
• Special Issues for the Compact Configuration. The compact configuration has a number of special issues which must be dealt with in the detailed array design:
• Antennas very close. Homogenous array mosaicing (Cornwell, Holdaway, and Uson, A&A 1993) requires that the antennas be fairly close together, so the effective Fourier coverage of the single dishes and the shortest interferometric baselines overlap. Simulations have indicated that the minimum distance suggested by Lugten of 1.3D results in acceptably good mosaic images, even when tapering the antenna illumination by 11dB.
• Complete Fourier Coverage. Mosaicing of arbitrarily complicated structure which fills the beam, will require complete effective Fourier plane coverage. Convenience in imaging large fields (ie, 100-1000 pointings) requires that the Fourier plane coverage be complete in a single snapshot. As the coverage departs from completeness, the image quality of very complex sources will gradually degrade.
• Shadowing. Any configuration compact enough to work well with mosaicing will also result in shadowing for low elevation observations.
• Beam Elongation. Sources at very low elevation will have very elongated beams unless the array is stretched north-south. However, an elongated array observing a low elevation source will have very different beam elongations at different hour angles, a potential problem for single snapshot mosaics.
• Reconfiguration Speed. We estimate that single antenna move times will be on the order of an hour if we design the antenna/station foot and cable mating for fast reconnection.

Configuration Development Plan

Compact Configuration

We are considering a high level design for a set of 3-4 compact configurations which will give excellent mosaicing (i.e., good short spacing coverage), minimum shadowing, and pretty good beam shapes for sources which transit as low as 10 degrees elevation. We plan on presenting detailed compact configurations and imaging simulations to verify their quality in early 2000.

Other Configurations

We are studying the effects of the Fourier plane distribution on imaging and observing strategy. We consider that the desired Fourier plane distribution is a matter open to debate. The largest configuration size and the number of configurations are also open to debate, subject to financial constraints. We hope to resolve the issues of the desired Fourier plane distribution, the largest configuration size, and the number of configurations with the help of the millimeter wavelength astronomical community shortly.

Revised by kweather@nrao.edu
January 23, 2000