Two types of polarisation products are generated by the pipeline. The first one being cubes of Stokes Q and U of 24 images each covering a bandwidth of 150 MHz resulting in a frequency resolution of 6.25 MHz. The second products are Stokes V images generated from the whole combined bandwidth of 150 MHz.
Due to the different characteristics of these products two different sets of metrics were used to validate their quality. These metrics are very similar to the ones used for the continuum validation, but needed to be adjusted due to the following reasons:
- Due to the physical nature of Stokes Q, U and V emission in the cubes can be a positive or negative quantity while it can only be positive in Stokes I.
- Image artefacts are mostly influenced by the instrumental leakage characteristics of the primary beam in combination with direction dependent effects. This means that the strongest artefacts often appear for sources far away from the beam centres where the instrumental leakage is higher.
- For Stokes Q and U even faint artefacts in individual images can stack up if those are present over the whole cube at a similar position once the RM-Synthesis technique is applied in later stages of the analysis. This is also possible vice versa where strong artefacts in an individual image can be averaged out in this process and lead to a good image quality after RM-Synthesis processing.
- Stokes V represents the circular polarisation. Astronomically circularly polarised sources are extremely rare and most often show percentage polarisation of below 1%, so that Stokes V images should normally be regarded as empty.
In the following we describe the sets of metrics and the determined values for the quality assurance of the polarisation cubes/images. All values and the validation information for released beams is available in the table of the survey data release for polarisation observations.
The following parameters were derived for each Stokes V image:
- σin: rms in the inner part of the image, which is defined as a radius of <30′ around the central coordinate of a beam [microJy/beam]
- σout: rms in the outer part of the image, which is defined as a radius of >60′ around the central coordinate of a beam [microJy/beam]
- FTmax: Absolute of the Fourier Transformation of the image. This value gives a good measure of repeating structures in the image such as amplitude artefacts which manifest as stripes.
- peakinner: The maximum of the absolute values of the inner part of the image as defined in σin (see above) [microJy/beam]
- bmin: The diameter of the FWHM minor axis of the synthesised beam [arcseconds]
A Stokes V image fails validation if one of the following criteria is met:
- σin and/or σout > 60 microJy/beam. This is the same value as for the continuum validation.
- bmin > 15”. This criterium filters any images for observations where failing or missing dishes are raising the FWHM of the synthesised beam above a value which is not acceptable for the survey.
- FTmax > 25. This value was empirically determined and filters any image, which is suffering from leftover RFI or strong amplitude errors.
- peakinner > 4mJy. Images with values above this limit are most likely strongly affected by instrumental leakage and in most cases also show strong artefacts.
The first two criteria match that of the continuum validation and are based on survey specification. The second two criteria were empirically determined to filter images with strong polarization artefacts.
Stokes Q/U cubes are validated depending on how many planes in a cube fail the following two criteria:
- The synthesised beam‘s minor axis of an image is larger than 17.5”. We are using a slightly larger value here compared to the continuum or Stokes V validation due to the fact that the beam becomes larger for images at lower frequencies.
- The rms noise in an image is larger than 300 microJy/beam. This corresponds to the same noise level as for the continuum and Stokes V images assuming a homogeneous noise distribution along the frequency axis. Stacking 24 images would reduce the noise by a factor of ~5.
Beams where more than one third of the images (>8) in Stokes Q or U are failing either of the above mentioned criterium do not pass the validation.
The table of released polarisation beams includes the following columns that describe the Stokes Q/U data quality:
Qbeam_frac: Fraction of Q image planes failing criterium 1. [0.0-1.0]
Ubeam_frac: Fraction of U image planes failing criterium 1. [0.0-1.0]
Qnoise_frac: Fraction of Q image planes failing criterium 2. [0.0-1.0]
Unoise_frac: Fraction of U image planes failing criterium 2. [0.0-1.0]