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Array Configurations with Uniform UV Coverage

John Bunton,
CSIRO Telecommunications and Industrial Physics, Australia

Introduction
The object of configuration This leads to a continental this poster is to free up thinking with regard to array design. In particular, the array can be asymmetric. the possibility of siting the central site of the SKA on coast and maximising the VLBI baselines.

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A full version of this paper submitted to Experimental Astronomy


Single Compact Component Designs
It is very likely that that the SKA will have a compact component of 1-3 km diameter containing almost half the collecting area. Adding a regular grid of antennas around the compact component gives a uniform UV coverage for correlations between the compact component and the grid of antennas. It is important to note that for these correlations an antenna at (x,y) with respect to the compact component generates the same information as one at (-x,-y). This means one half of the surrounding grid can be removed. In general: With a single compact component, remote antennas of a given baseline length can be restricted to a 1800 arc length. Removing half the surrounding grid of antennas gives the antenna configuration is shown in Fig 1. If the spacing of the grid is equal to the diameter of the compact component then: Complete sampling of the UV plane is achieved with a sampling interval equal to the diameter of the compact component.
gives very high quality snapshot imaging

Antenna configuration

Regular grid of N antennas with spacing D

Figure 1 Asymmetric Antenna Configuration with single compact component

2N antenna Compact component Diameter D

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Adding the correlations between the antennas of the grid does not greatly affect the sensitivity. If the grid is used to increase the maximum base line to 10 times that of the compact component and there are about twice as many antennas in the compact component. Assume N antennas in the grid and 2N in the compact component then the distribution of correlations is: Correlation type Compact - compact Compact - grid Grid-grid Number 2N2 2N2 0.5N2 Short baselines 80% of Long baselines 20% of Long baselines

It is seen that the grid-grid correlations contribute only 20% of the sensitivity and 75% of that is in common with the compact-grid correlation. Correlation between remote antennas can be largely ignored in designing the configuration. This greatly simplifies the design. configuration in Fig 1 is shown below. The resulting UV coverage for the

UV coverage

Compact/Grid Correlation

Figure 2 Simplified UV coverage for antennas shown in Figure 1

Compact/Compact Correlation

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Examples of Multiple Compact Component Designs
If there are multiple compact components then uniform coverage for compactgrid correlation is still possible. Cases for 2,3 and 4 components are shown below. In the limit as the number of compact components increases, the roles of the grid of antennas and compact components reverse. Long baselines occur between compact components. The antenna of the regular grid provide short baseline correlations.
Antenna configuration
Regular grid of antennas One of 2 Compact Components

UV coverage

Regular grid of antennas

One of 3 Compact Components

Figure 3 Configurations with 2, 3 and 4 compact components

One of 4 Compact Components

Regular grid of antennas

Figure 4 Examples of antenna configuration with a large number of compact components. Asymmetric and Spiral forms shown

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