Survey Quality

Jim CondonNRAO, Charlottesville

Survey Qualities

Leiden 2011 Feb 25

Point-source detection limit Slim

Resolution Ωs

Brightness sensitivity Tlim S∝ lim / Ωs

Frequency coveragePolarizationArea of sky covered

Accuracy:flux densities

positionspolarizationimage fidelityuniform 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., VNVSS/VEMU ~ 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 rangeMedian <Seff> ~ 0.16 Jy x ×Ω(deg2)

[Minimum Seff > 0.06 Jy x ×Ω(deg2)]

Ex: EMU σ~ 10 μJy so

DR ~ 16000 ~ 42 dbEx: SKA σ~ 9 nJy so

DR ~ 73 db

Leiden 2011 Feb 25

Calibration errors and dynamic rangeAssume that an image is dominated by a single strong source

Let ε = antenna-based rms intensity calibration error ΔS/SLet 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 < 20000D < M1/2 N / ϕ

(Perley 1988, in “Synthesis Imaging in Radio Astronomy”, p. 287)

SKA Workshop 2009 Feb 19

Pointing errors, beamshapes, and

intensity calibration errorsσ2 = 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

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

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” < 16% (P > 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 ‘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

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