The Aperture Array approach for the Square Kilometre Array Concept Extensions and Response to Questions

The European SKA Consortium has adopted the aperture array concept to tackle the decametre and longer wavelength universe. The lower frequency falls in the high frequency LOFAR band while its highest frequency is close to the OH rest frequencies. In this concept beam pointing to the radio-source is electronically controlled over a large scan-angles while interferers can be suppressed adaptively. By multiplying parts of the electronic subsystems by, say, M, simultaneous and independent reception of widely separated directions is possible without the need to multiply the basic mechanical structure. This in effect, corresponds to creating multiple i.e. M, Fields Of View. For example for HI surveying, this capability enhances the mapping speed in proportion to M. The baseline design as proposed here assumes 8 independent FOV´s thereby extending the range of science capabilities. This may be compared to common reflecting telescope systems in which beam pointing is done mechanically of the whole telescope and its single field of view is determined by the diffraction limit of the telescope. In the aperture array concept, the aperture field of the incoming radiation, is sampled by a 2 D array of independent receivers at close range such that the distance between neighbouring elements is half a wavelength at the highest frequency only. Hence the name dense aperture array. To compensate for the increasing Galactic noise to lower frequencies, a three band array has been chosen with increasingly larger collecting area. Table 1.1 below summarizes the pertinent performance parameters of the aperture array concept. * Collecting Area 3.5x 10 6 m 2 in three bands 2x 10 6 m 2 (low), 0.98x 10 6 m 2 (mid), 0.52x10 6 m 2 (high band) * Number of stations 100 stations but concept supports more stations * Station diameter for each band 160m (low), 112m (mid), 81m (high), co-located at station site * Mount Fixed mechanical mount. * Scan angle azimuth/elevation Electronic: +/-45 deg.. Larger range possible with scan anomalies * T sys (K) @ (GHz): 535 (0.12), 95 (0.3), 45 (0.6), 35 (1-1.5GHz) * Efficiency (estimate) ? = A eff /A physical 0.8 * A eff /T sys (x 10 4 m 2 /K) per beam @ (GHz) 0. * Field of View (determined by patch size) 260-42 deg 2 (low band), 65-10.5 deg 2 (mid and high band) * Angular resolution 0.01 arcsec @ 1.4 GHz * Instantaneous BW: 180 MHz/pol. (low), 0.2 GHz/polarisation …

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