[asac] [almanews] ALMA Memo No. 348 Released

[Carolyn White]

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.


The full html version of ALMA Memo #348  is at URL:

    http://www.alma.nrao.edu/memos/html-memos/alma348/memo348.html


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