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Survey Quality

Survey Quality

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Survey Quality

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  1. Survey Quality • Jim Condon • NRAO, Charlottesville

  2. Survey Qualities Point-source detection limit Slim Resolution Ωs Brightness sensitivity Tlim ∝ Slim / Ωs Frequency coverage Polarization Area of sky covered Accuracy: flux densities positions polarization image fidelity uniform sensitivity Leiden 2011 Feb 25

  3. Random and Systematic Errors Radiometer equation: σ ∝ (B t)-1/2 “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” - Archimedes But V ∝ (B t)+1/2 e.g., VNVSS/VEMU ~ 1/60 Leiden 2011 Feb 25

  4. Systematic Errors: EMU Leiden 2011 Feb 25

  5. Survey Dynamic Range Leiden 2011 Feb 25

  6. Effective flux density in primary beam for calculating dynamic range Median <Seff> ~ 0.16 Jy x ×Ω(deg2) [Minimum Seff > 0.06 Jy x ×Ω(deg2)] Ex: EMU σ~ 10 μJy so DR ~ 16000 ~ 42 db Ex: SKA σ~ 9 nJy so DR ~ 73 db Leiden 2011 Feb 25

  7. Calibration errors and dynamic range Assume that an image is dominated by a single strong source Letε = antenna-based rms intensity calibration error ΔS/S Let ϕ = antenna-based rms phase error (radians) N = number of antennas M = number of independently calibrated observations D < 2 M1/2 N / ε ex: M = 8, N = 36, ε ~ 0.01 → D < 20000 D < M1/2 N / ϕ (Perley 1988, in “Synthesis Imaging in Radio Astronomy”, p. 287) SKA Workshop 2009 Feb 19

  8. Pointing errors, beamshapes, and intensity calibration errors σ2 = rms pointing error (2 dimensions) θ = HPBW ε = ΔS / S Single source on axis: ε ≈ 4 ln(2) (σ2/θ)2 Distributed sources: ε ≈ 4 [ln(2)/π]1/2(σ2/θ) (see SKA Memo 114) SKA Workshop 2009 Feb 19

  9. Calibration suggestions for SKA pathfinder surveys • Grid map of strong unresolved source to fully sample the primary beam(s) • Develop and test analysis package using real data in a small area of sky (6 months on 1 hour of data for the NVSS) • Check results on faint sources using better reference images with slightly higher angular resolution; calculations alone are not reliable • Calculate quantitative hardware requirements before it is too late • Antenna pointing accuracy (gravitational, thermal, wind) • Accuracy and stability of PAF primary beams • Pointing correction algorithms • PAF element weighting for “best” primary beams (highest G/T? lowest sidelobes?) Leiden 2011 Feb 25

  10. Unexpected Errors Leiden 2011 Feb 25

  11. Systematic errors in faint-source counts P(D) 1.4 GHz Huynh et al. (2005)

  12. More of the same

  13. Anisotropic source distributions?

  14. 1.4 GHz source counts in 17 FLS fields • 2 is consistent with no clustering (P > 30%) • “Cosmic rms” < 16% (P > 99%)

  15. The fault, dear Brutus, is not in our stars,but in ourselves… • Large count corrections for missing extended sources in all flux bins • Deconvolution errors (CLEAN bias, overcleaning) • Primary beam shape, pointing errors • RMS errors in integrated flux densities of ‘5σ’ sources ~ 21/2σ, not stable and not well understood • Greed • …? Recommendation: Use a reference field with higher resolution and lower noise to verify calculated errors of source fluxes, sizes, and positions. Leiden 2011 Feb 25

  16. Astronomical community:Don’t create a mob of angry villagers Leiden 2011 Feb 25

  17. All’s well that ends well The whole astronomical community should benefit “The NVSS team members have agreed to use only these electronically released results for their own research.” Easily understood and uniform survey parameters (sensitivity, resolution, position accuracy, no holes,…) Easy-to-use web site Leiden 2011 Feb 25