POSSUM Polarisation Catalogue

Status page for the level-5/6 catalogue for the ASKAP POSSUM Survey.

The POSSUM Polarisation Catalogue is created automatically by the ASKAPSoft system to produce the level-5 data products. Level-5 data products are released as level-6 after a a quality control step. This quality control step is limited to setting flags based on the information available in the level-5 data.

Note: By the end of 2014 it had become clear that significant numbers of sources detected in POSSUM will be resolved (>10 arcsec). A 3x3 or 5x5 pixel extraction box will not be sufficient to extract a representative polarised spectrum to make a RM-grid. Please comment and vote on the 5 proposed source extraction schemes on this page.

The Prototype PPC software

Prototype RM-pipeline has been built by Cormac Purcell.

  • The core code and simple utilities are available from GitHub.
  • The end-to-end pipeline is available from GitHub, but as of 2017-Feb needs to be updated with latest core code.
  • The algorithms and table format are described in POSSUM report 62.

See the SG5 page for more information.

The table below summarises the status of the PPC data-products and notes open questions.

PPC version 2014: Operates on 1D spectra extracted from compact sources

Full technical description is summarised in POSSUM Report 62





Stokes I, Q, U Spectra

1D spectra extracted at the location of compact EMU sources.

May need to be updated to deal with resolved sources.


I, Q, U Noise Spectra

1D spectra of RMS noise from a square aperture of 50 beam areas. Use MADFM estimator to be robust to source contamination.

May need to be updated to deal with resolved sources.


Model Stokes I spectrum

1D model Stokes I spectrum used to create fractional polarisation spectra q=Q/I, u=U/I.

Constructed using a 2nd order polynomial fit. If fit fails default to flat spectrum at median flux.


lambda_0, freq_0

Weighted average wavelength squared and frequency squared.

Used to convert FDF back from fractional units to Jy.



Rotation Measure spread function and width of main lobe.

Property of wavelength sampling and weighting (assumed 1/variance^2).



Dirty Faraday Dispersion Function

Real, Imaginary & Polarised Intensity Faraday Dispersion Function. Created using fractional Q/I and U/I spectra, weighted by variance.


FDF Measurements

RM of peak in intensity FDF, fractional polarisation of peak (+ debiased version), polarisation angle at lambda_0, polarisation angle at lambda=0 (derotated), complexity flags.

Complexity flags measure deviation of P spectrum from a constant and residual in Q and U after subtracting a Faraday thin component at the peak RM.


Quality Flags

Flags indicating something wrong with the data or processing.

Stokes I model fit failed, unusually high noise, large numbers of channels flagged, RM unusually high, detection on the edge of the FDF or data cube.

In Progress

Questions, discussion points and notes.

  • Need to supply unit-check (artificial) data with the PPC prototype for verification purposes.
  • Do we need a RM-spectrum in annulus/region around EMU source.?
  • Is a noise 'map' in I, Q, U & V calculated along channels useful?
  • Complexity: If using spread in CC to determine complexity, need to have 20-30 channels across RMSF main lobe. Better approach is to look for variations is P vs lambda^2.
  • Specification of algorithms/metrics for quality control. Key step in moving from Level 5 to 6.
  • Can we use EMU MFS spectral index and curvature as a model for Stokes I data?
  • Input POSSUM data cubes (300 channels) won't be checked by EMU. Need to create a quality control process for these (reports 45 and 51).
  • How do we detect sources that are between 5 and 10 sigma (90% of possible detections)?
  • Would bright sources be leakage dominated? How to incorporate leakage in uncertainties?
  • If we provide a RM-synthesis cube, how can we correct for spectral index over the whole image? What do we divide by for the 'noise' pixels. Larry says we should use the fitted spectrum from MFS and divide every off-source pixel by that spectrum.

Relevant Reports

#MilestonePerson(s) ResponsibleWGDue Date
3Draft data verification plan, including list of automated tests to be applied to data: reportHarvey-Smith / S. Brown515 Feb 2011
8Draft specification of interface between POSSUM WGs 2/5/8 (including catalogue entries and how to calculate them): reportS. Brown / Harvey-Smith2/5/828 Feb 2011
23Final data verification plan, including list of automated tests to be applied to data reportHarvey-Smith530 Aug 2011
24Final specification of interface between POSSUM WGs 2/5/8 (including catalogue entries and how to calculate them) reportS. Brown2/5/815 Jun 2011
32Specifications for POSSUM Polarisation Catalogue (PPC) & POSSUM Polarisation Atlas (PPA)Gaensler / Rudnick5 30 Sep 2011
56Outputs of POSSUM pipeline (verified data products)Harvey-Smith / WG551 August 2012
62Report on integration of polarisation pipeline for BETA. Description of prototype PPC software pipeline. reportCormac Purcell2/5/825 Nov 2014

Important Resources

PPC Source Extraction Vote 2015Options to advance beyond the processing 1D spectra that would allow the characterisation of extended sources.
POSSUM Workshop 2013, TalksEMU
POSSUM Workshop 2012Early thoughts on limits of RM-synthesis and the state of source-finding were presented in the 2012 POSSUM workshop