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SKA Memorandum #3
SKA Technical Specifications Ron Ekers December 2001 The SKA specs have been evolved in a series of workshops over many years and in some cases the origin of the various specifics has been lost. In order to provide input to the science and engineering groups now looking at the scientific drivers and the specification trade-offs, I have provided this somewhat personal review of how we got to the present set of specifications. The specifications I have used and their definitions, which are repeated here, are from the SKA Science Case, p17. A_eff/T_sys (2 x 10⁴ m²/K) The effective collecting area divided by the system temperature. This may be a function of frequency. • Sets the point source sensitivity and corresponds to 1 square kilometre collecting area, eg Aeff = 50% total aperture with Tsys = 25K. It is formulated this way to include the differences in aperture efficiency, and to allow different technologies to trade effective area for Tsys . • The spec is set by the HI brightness sensitivity at a moderate spectral resolution (spectral resolution of 10⁴ corresponding to 30 km/sec). This enables detection of a normal galaxy like M101 at any z by using HI up to z = 4 and CO at any z > 4. • For continuum, this will allow detection of a normal galaxy to z = 8 in non-thermal emission – and beyond z = 8 as the thermal emission is redshifted into the SKA band. • A secondary motivation is to have a sensitivity at least 10-100 times any existing telescope. Total Frequency Range (0.03 – 20 GHz) 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. • A square km of collecting area is expensive (at frequencies above 150 MHz) so we need the widest possible frequency range to maximise value. • At frequencies below 150 MHz the collecting area doesn’t dominate cost - this is the LOFAR range. • High frequencies are required to access the thermal universe and for VLBI resolution and penetration. There will be a major cost versus max trade-off. For almost any design $  n , with n ~1 for dishes, n ~ 2 for phased arrays. Imaging Field of View (1 square degree @ 1.4 GHz) 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. • Set by HI surveys using spectral line correlator imaging. Number of Instantaneous Pencil Beams (100) The number of ‘phased array’ pencil beams that can be placed simultaneously within the Imaging Field-Of-View, FOV, for point source observations such as pulsars, stars (including SETI), and VLBI. • These are beams formed by combining all the collecting area (or a significant fraction of it). Most of these beams will be used for temporal signal processing at full sensitivity (VLBI, pulsars, GRB, IDVs, spacecraft communication, SETI). A smaller number (not specified) may support imaging correlators. The actual number is fairly soft and may be unreasonably high. Angular Resolution (0.1 arcsec @ 1.4 GHz) The maximum angular resolution of the array as determined by its largest linear extent (longest baseline). • Minimum acceptable is set by continuum confusion at 1.4 GHz. • Maximum is set by VLBI science. • This, together with the Surface Brightness Sensitivity, determines the overall radial distribution of antennas. This does not have to be even close to uniform. Number of Spatial Pixels (10⁸) The number of spatial resolution elements in a map synthesized within the Imaging Field-Of-View. • Set by the minimum angular resolution (~1” at 3 pixels / beam) for HI imaging over the full Field-Of-View. Note that the combination of the highest resolution and the full FOVwould require much larger images but this is not a specification as astronomical drivers for the higher resolutions don’t require the full FOV. Surface Brightness Sensitivity (1 K @ 0.1 arcsec [continuum]) The minimum detectable (5) continuum surface brightness for a specificed resolution, eg, 1 K @ 0.1 arcsec. This may be a function of frequency. • Determined from the Aeff/Tsys , bandwidth and resolution. Instantaneous Bandwidth (0.5 + /5 GHz) 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. • More instantaneous bandwidth means more continuum sensitivity and better spatial coverage (bandwidth synthesis). More is better for all continuum applications as long as it can be broken into enough channels to avoid bandwidth smearing. • The maximum will be set by technical considerations such as cost and RFI. Clean Beam Dynamic Range (106 @ 1.4 GHz) The best intensity dynamic range that may be obtained in a fully processed synthesised map, as limited by unknown errors in the array or its environment. • Set by the ratio of the strongest continuum source in the Imaging Field-Of-View at 1.4 GHz to the weakest detectable source. The required dynamic range will decrease with Field-Of-View and hence with increasing frequency. Polarization Purity (-40 dB) The error in Q, U, and V Stokes parameters as a fraction of I for a strong radio source after all data processing, as limited by unknown errors in the array or its environment. • Set by the polarization precision needed for weakly (<1%) polarized sources.