SKA Specifications and Reflector Antennas P. Dewdney Mar 31, 2008

SPDO Outline 1. SKA Top-Level Specifications. 2. Potential Implementations for Low-Mid Frequency SKA (Phase 2). 3. Implications for reflector antenna design. 4. Snapshot of current antenna design activity.

SPDO Remit - “Specifications Tiger Team” Consider science-engineering trade-offs –Current knowledge of likely key technologies. –Evolution of technology in next 5-10 yrs. –Cost at time of construction. Multi-Phase Construction for SKA Proposed –Phase 1  First stage construction 0.07 – 10 GHz  Low band (70 – xxx MHz)  Mid-band (xxx – 10 GHz)  xxx = MHz, depending on realization.  300 M€ (200 for telescope, 100 for “infrastructure”) –Phase 2  Build-out of Phase 1 to full collecting area.  1500 M€ (1000 for telescope, 500 for “infrastructure”)  Construction end –Phase 3  High-band SKA – not yet well defined, except frequency coverage to ~35 GHz.  Construction start ~2022. Develop “top-level” SKA specifications

SPDO Desired SKA Specifications Summary of Science Case This is a very refined list – many possibilities are NOT included!

SPDO Key Figures of Merit SKA is a high sensitivity array. Two most important measures of success: –A eff /T sys = “staring” sensitivity  Emphasizes imaging sensitivity when the position of the source is known. –(A eff /T sys ) 2 FoV = Survey Speed (FoM)  FoV = instantaneous field-of-view.  Emphasizes search mode sensitivity.  Imaging area >> FoV.

SPDO SKA Performance Measures Performance MeasureCost Driver Frequency Range (>2.5 Decades)Major Point Source Sensitivity (A/T sys )Major Survey Speed (A/T sys *  ) Major Imaging Dynamic Range (>10 7 )Medium-Major Spectral Dynamic RangeMinor Polarization Purity (10 3.x )Medium Configuration (0 – 10 3.x km)Medium Instantaneous Bandwidth (dependent on ) Medium Number of Frequency Channels (32,000)Major Total Power Calibration (5%)Minor Time-domain Capabilities (transients, pulsars) Minor Sky CoverageMedium

SPDO Representative Phase-2 Implementations a b c 15-m dishes MHz

SPDO SKA Spec’s Relevant to Reflector Antennas 1. Frequency Range Possibly 0.3 – 10 GHz (baseline option), depending on success of Aperture Arrays. 2. Continuum imaging dynamic range (1 st Galaxies & Black Holes) Artifact level must be lower than thermal noise (8 nJy/beam) in ~400 hr at BW = 400 MHz. Approx. 1 bright source (~80 mJy) per deg 2 at 1.4 GHz => 60 dB dynamic range for 10-m dish. Sources in random positions wrt to beam center. Larger FoV requires greater dynamic range. FoV (even PAF individual beams) scales with 1/ => increased dynamic range. Source spectra also contribute to increased dynamic range requirement at low frequencies: sub 1mJy sources statistically have flatter spectra because they are more likely to be star-forming galaxies. k-correction pushes highly redshifted non-thermal spectra of galaxies to lower frequencies. 3. Polarization Purity Pulsar timing observations affected by leakage of Q, U into I. Spec of –30 dB after calibration. For pulsar timing, the spec need be met only at the beam center. For continuum surveys, a similar spec may be needed over the entire field. 4. Noise & Systematic Errors Spillover, losses, and scattering add noise; non-random effects will have worse consequences. 5. Sky Coverage El > 10 deg is probably adequate. 6. Slew Speed: requirement depends on mapping strategy.

SPDO SKA Spec’s Flow-down to Antennas 1. Frequency range Diameter sufficient to meet dynamic range specs at 300 MHz. Low spillover of primary and/or secondary. Optical configuration may have to include both prime and secondary foci. 2. Polarization Purity Many of the dynamic range issues also apply to polarization. See next slide. Feed design will be the strongest influence. Offset antennas have polarization pattern when fed from the prime focus. Effect can be cancelled at the beam center with a secondary reflector.

SPDO Spec’s Flow-down (cont’d) 3. Imaging dynamic range Stability of polar diagram on sky – rotation on sky is a subject of debate. Stability of polar diagram on ground – very small time-variable signals. Can’t have it stable on the ground and on the sky. “Recovered” Pointing Error Strong sources near ½ power point very sensitive to pointing (  P  0.72 [  /FWHM] for Gaussian). For  P < 10 -6,   1.4 X FWHM. Clearly this “spec” cannot be met without recovery of pointing from the data. Self-calibration, mosaicing and other “averaging” techniques will be necessary to effectively recover pointing errors (e.g. see Perley, Synthesis Imaging in RA, 1999). System modelling and testing with existing telescopes will be needed. “Working spec” of 0.01 FWHM at 10 GHz might be reasonable for now. Scattering Feed leg and focus box scattering. Surface imperfections. Rotating or varying scattering patterns on the sky will likely be a major problem. Sidelobes Even perfect diffraction sidelobes will be difficult to handle if they vary against the sky. PAF’s offer a chance to optimize feed pattern to reduce sidelobes while maintaining good efficiency at low frequencies.

SPDO Remarks on Antenna Spec’s We can’t afford poor-quality antennas. The cost of mitigating the effects of uncorrected errors induced by antenna imperfections could be larger than the cost of making the antennas better. Careful modelling and simulations will be needed to understand this better. We may have a choice between “sky-mount” and clear-aperture antennas. How do we investigate this choice? Clear-aperture antennas might be allowed to rotate against the sky. Can polar patterns be made sufficiently symmetrical or with known asymmetry?

SPDO Role of Pathfinders is Critical Provides basis for building antennas in medium quantities –Justifies expenditure on medium-scale production technology.  Apply the “1, 10, 100” concept. –Provides a means of assessing subtle areas of performance, especially dynamic range.  Sufficiently large array is necessary to obtain sensitive tests. Important that Pathfinders provide scope for innovation –High enough quality to be able to meet SKA specs. –Scope for design and production R&D.

Patriot Antenna (12 m) ATA Antenna (6 m) KAT Antenna (15 m) DRAO Antenna (10 m)

Composite Dish Manufacturing - meerKAT Dish is structural - very simple backup structure Manufacturing process: Machine patterns (composite pattern material) Make composite moulds of the patterns Combine composite moulds to create dish mould Innovative and cheap honeycomb structure Vacuum Infusion process used for molding entire dish as a unit On-site manufacturing – open environment for prototype Achieved 1.5 mm rms; 1.0 mm rms quite feasible Courtesy Anita Loots

rms error: 0.25 mm Final Mould Alignment Composite Dish Manufacturing - DRAO Removing from Mold Mounted on Drive rms error: 0.25 mm

SPDO Metal Dish Manufacturing ATA 6m hydro-formed (~20 GHz) Patriot/ASKAP 12m variant (test antenna) (includes “conventional” feed rotator) Patriot/JPL 12m stretch-formed panels (~32 GHz)

SPDO Considerations for TDP Antenna Development Parameterized antenna model suitable for insertion into an end-to-end model of the SKA. –help refine antenna spec’s. Develop a refined antenna cost model. Set up system for field evaluation of antennas –Basic measurements – pointing, slewing, etc. –  T from 5C to 55C in 2 hours, sun shining on one side of dish. –Wind distortion. Build antenna on VLA site –to carry out tests with VLA.  Dynamic range, imaging, calibration, etc. Wide-band feed/rcvr refinements and testing.

SPDO End