Controlling the masks used by pol2map

The map-making process used by the pol2map command uses two masks, each of which divides the field up into source and background regions:

  • The ‘AST’ mask: this is used to define the background regions that are to be forced to zero after each iteration of the map-maker algorithm (except the last iteration). This form of masking helps prevent the growth of artificial large scale structures within the map. Any real astronomical signal present within the masked background regions will tend to be suppressed in the final map, so it is important that the AST mask correctly identifies regions of significant emission down to a low level.
  • The ‘PCA’ mask: this is used to define the source regions that are to be excluded from the Principal Component Analysis. This analysis is used to remove the correlated backgrounds in the bolometer time stream data. The time-stream data for astronomical sources are not correlated across bolometers, and so tend to disrupt the PCA. For this reason source regions are excluded from the analysis.

Two separate masks are used because experience has shown that disruption of the PCA is caused mainly by the brighter central source regions. Consequently, the source regions within the PCA mask can  be smaller than the source regions within the AST mask.

A total intensity map of DR 21 showing the AST mask in green and the slightly smaller PCA mask in blue.

Default masks are created automatically by pol2map in a manner specified by the MASK parameter.

  1. On ‘step 1’ of a typical POL2 data reduction, MASK is left at its default valuer of “Auto”, causing new masks to be generated automatically at the end of each iteration of the map-making algorithm. This ‘auto-masking’ process identifies an initial set of sources by thresholding the current map estimate at the SNR value specified by configuration parameter “xxx.ZERO_SNR”, where “xxx” is either “AST” or “PCA”, depending on which mask is being generated. Each of these initial source regions is then expanded to include adjoining pixels down to the SNR level specified by configuration parameter “xxx.ZERO_SNRLO”.
  2. On ‘step 2’ and ‘step 3’ of a typical POL2 data reduction, MASK is set to the coadd of all the total intensity maps created at step 1. The pol2map script first creates a pair of AST and PCA masks from the supplied coadded map and then uses these masks on all iterations of the map-making algorithm. The findclumps command in the Starlink CUPID package is used by pol2map to create the masks. The process used by findclumps is the same as describe above for step 1 – initial sources are defined by a fixed SNR threshold within the supplied coadd and these are then extended down to a lower SNR threshold. The pol2map command sets these threshold values to the values of the four configuration parameters listed above – “AST.ZERO_SNR”, “AST.ZERO_SNRLO”, “PCA.ZERO_SNR” and  “PCA.ZERO_SNRLO”.

These configuration parameter all default to the following values specified in the pol2map script:

AST.ZERO_SNR = 3 
AST.ZERO_SNRLO = 2 
PCA.ZERO_SNR = 5 
PCA.ZERO_SNRLO = 3

If you wish to investigate the effects of changing these values, you should supply new values using the CONFIG parameter of the pol2map command. For instance:

% more conf
ast.zero_snr = 2.5
ast.zero_snrlo = 1.5
% pol2map config=^conf

Any values not specified will retain the default value listed above.

Note, prior to 10th July 2019 the above method could only be used at step 1 (the supplied settings were ignored if supplied at step 2 or 3). Later versions of pol2map do not suffer from this problem – the supplied values are honoured at all steps.

Footnote – for completeness it should be mentioned that the  COM and FLT models are also masked, in addition to the AST and PCA models. At step 1 (the auto-masking stage) the masking of COM and FLT is controlled by a similar set of configuration parameters to AST or PCA, except that “XXX” becomes “COM” or “FLT”. At steps 2 and 3 (the external-masking stages), the COM and FLT models are masked using the PCA mask generated by findclumps, and so COM and FLT masking cannot be controlled independently  of the PCA mask.

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