Survey Quality Jim Condon NRAO, Charlottesville

Survey Qualities Leiden 2011 Feb 25 Point-source detection limit S lim Resolution Ω s Brightness sensitivity T lim ∝ S lim / Ω s Frequency coverage Polarization Area of sky covered Accuracy: flux densities positions polarization image fidelity uniform sensitivity

Random and Systematic Errors Leiden 2011 Feb 25 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., V NVSS/ V EMU ~ 1/60

Systematic Errors: EMU Leiden 2011 Feb 25

Survey Dynamic Range Leiden 2011 Feb 25

Effective flux density in primary beam for calculating dynamic range Median ~ 0.16 Jy x × Ω (deg 2 ) [Minimum S eff > 0.06 Jy x × Ω (deg 2 )] Ex: EMU σ ~ 10 μ Jy so DR ~ ~ 42 db Ex: SKA σ ~ 9 nJy so DR ~ 73 db Leiden 2011 Feb 25

Calibration errors and dynamic range Assume that an image is dominated by a single strong source Let ε = 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 M 1/2 N / ε ex: M = 8, N = 36, ε ~ 0.01 → D < D < M 1/2 N / ϕ (Perley 1988, in “Synthesis Imaging in Radio Astronomy”, p. 287) SKA Workshop 2009 Feb 19

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

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

Unexpected Errors

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

More of the same

Anisotropic source distributions?

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

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 ‘5 σ ’ sources ~ 2 1/2 σ, 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

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

All’s well that ends well Leiden 2011 Feb 25 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