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ALMA Memo #348
A Preliminary ALMA Zoom Array Design for the Chanjnantor Site.

John Conway
February 15, 2001

I present a preliminary zoom array design for ALMA fitted into the the Chajnantor site, and compare its properties with the proposed double ring strawperson array. Both arrays have virtually the same number of pads and require almost the same number of antenna moves. To first order the two arrays are shown to have similar radial distributions of baselines, although the coverage of the zoom array is smoother and is slightly more centrally condensed. For snapshots the peak sidelobes for the two styles of array are similar. However the zoom array has 40% smaller peak sidelobes for long track observations. For both snapshots and long tracks the degree of uv cell occupancy for the two arrays is very similar. We argue that zoom arrays will have slightly smaller or equal phase noise efects than dual ring arrays of the same resolution. For the zoom array we show that North-South extended array configurations with low sidelobes exist for observing Northern hemisphere or South polar sources.

The zoom array is a very flexible design, which can accommodate many different modes of operation. It can for instance be operated as set of fixed arrays like the double ring, with the important difference that the sizes and numbers of configurations are variable. Alternatively the array could be operated in a continuous reconfiguration mode with perhaps six antennas being moved per week. This mode of operation has some astronomical advantages and significant operational advantages. However the array is operated, if antenna moves are prevented by bad weather, the zoom array is aways left in a configuration with a good uv coverage without stranded 'outlier' antennas.

After a certain amount of convergence the two styles of array proposed for the intermediate configurations are no longer dramatically different. However in almost all performance areas we argue that the zoom array is slightly superior. This combined with its much greater flexibility suggests that the zoom array design should be chosen for the intermediate configurations. Put another way - we find no strong argument in favour of (and several against) building particular scale lengths into the array design as is done in the dual ring design.

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Last modified: February 28, 2001

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