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The SKA Concept
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| Over the past several years, discussions have been occurring
in several countries about the next logical step in radio astronomy following
the up-coming construction of large millimetre arrays. An initiative
has emerged to develop a telescope to provide two orders of magnitude increase
in sensitivity over existing facilities at metre to centimetre wavelengths.
To achieve this goal will require a telescope with one square kilometre
of collecting area - one hundred times more collecting area than the Very
Large Array. The Square Kilometre Array (SKA) would probe the gaseous
component of the early Universe, thereby addressing fundamental questions
in research on the origin and evolution of the Universe. The SKA would
complement planned facilities at other wavelengths, such as the Next Generation
Space Telescope (NGST). HI, CO and continuum radiation would be observed
from the interstellar media of most of the galaxies the NGST will discover
in the infrared at large redshifts. Extensive discussion of the envisioned science drivers and of the evolving technical possibilities has led to a concept for the Square Kilometre Array and a set of design goals. The SKA will be an interferometric array of individual antenna stations, synthesizing an aperture with diameter of approximately 1000 km. The 106 square meters of collecting area is distributed over 30 interferometric stations. Each station is a 200 metre diameter telescope. Approximately 80% of the array is contained within a centrally-condensed inner array to provide ultrahigh brightness sensitivity at arc-second scale resolution for studies of the faint spectral line signatures of structures in the early Universe. The outrigger stations provide a ten-fold increase in angular resolution to allow high resolution imaging of faint emission from the interstellar media of distant galaxies. At a SKA technical workshop in Sydney , Australia in December 1997, the goals for the basic system parameters were established. The array will blend basic interferometric techniques with multi-element, phased receiving systems. consequently, some of the terminology used, particularly that related to the beam-forming hierarchy, is non-intuitive. For the sake of clarity a short technical explanation for all of the listed parameters is provided below. SKA Design Goals
Aeff /Tsys : The effective collecting area divided by the system temperature. This may be a function of frequency. Total Frequency Range: The total frequency tuning range of the instrument. This may be divided into sub-ranges with different antenna technologies, and it is not necessarily contiguous. Imaging Field-of-View: The instantaneous, contiguous solid-angle area of the sky that can be imaged, given a sufficiently capable correlator. This area will be a function of frequency. Number of Instantaneous Pencil Beams: The number of "phased array" pencil beams that can be placed simultaneously within the Imaging Field-of-View for point source observations such as pulsars, stars (including SETI), and VLBI. Maximum Primary Beam Separation: This specification assumes that the facility will have at least two levels of beam forming. Signals from small antennas (dipoles, small dishes, etc.) are combined to form an array element primary beam, and signals from array elements can be combined in a correlator to make a map within the primary beam or combined directly to form one or many pencil beams within the primary beam. More than one primary beam could be formed within the pattern of the small antennas. The Maximum Primary Beam Separation specifies how far apart these primary beams can be formed simultaneously. Angular Resolution: The maximum angular resolution of the array as determined by its largest linear extent (longest baseline). Surface Brightness Sensitivity: The minimum detectable continuum surface brightness for a specified resolution, e.g., 1K @ 0.1 arcsec. This may be a function of frequency. Instantaneous Bandwidth: The widest contiguous frequency range that may be observed simultaneously given enough correlator or other processing capability. Typically this means the widest selectable IF filter bandwidth before the digitizer. Number of Spectral Channels: The number of independent frequency samples from the array after all signal processing. Number of Simultaneous Frequency Bands: The number of widely spaced frequency ranges that may be observed simultaneously. for example, a stellar flare study might want to observe at 1.4 and 5.0 GHz at the same time, each with instantaneous bandwidths of 0.3 GHz. Imaging Dynamic Range: The best intensity dynamic range that may be obtained in a fully processed synthesized map, as limited by unknown errors in the array or its environment. |
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