Investigating the effects of uncertainties in the POL2 IP model

Vector maps created from POL2 data need to have the instrumental polarisation (IP) removed to be useful. Determining a good model for the IP has been a challenge, and not surprisingly the currently accepted model has somewhat uncertain parameter values.  These uncertainties in the IP model translate into uncertainties in the final vector maps in complicated ways that cannot easily be estimated analytically. However, a possibly approach to estimating the uncertainties in the vector map is to produce several vector maps using slightly different IP models and then look at the differences between the resulting maps.

The pol2map command provides two options for doing this:

  1. The level of IP – as a percentage if the incoming total intensity – removed by the IP model can be raised or lowered by a specified amount from its default value. To do this, use configuration parameter “ipoffset“. For instance, to create a vector map using an IP level that is 0.3% greater than the default, do:

    % cat conf
    ipoffset=0.3
    % pol2map config=^conf ...

  2. The IP removed by the IP model can be rotated by a specified angle from its default orientation. To do this, use configuration parameter “ipangoff“. For instance, to create a vector map using an IP at an angle of 0.5 degrees to its default angle, do:

    % cat conf
    ipangoff=0.5
    % pol2map config=^conf ...

The above changes should be applied when running pol2map to create the Q and U maps, together with the final vector catalogue (“step 3”).

Previous investigation of the IP model suggests that the above values for ipoffset and ipangoff (i.e. 0.3 % and 0.5 deg.) are reasonable estimates of the uncertainties in the IP model parameters. Rotating the IP by up to 0.5 degree should usually have little effect on the final map, so it is probably  reasonable to ignore ipangoff and concentrate on  the effects of changing ipoffset. A possibly strategy is to use pol2map (step 3) three times to generate three sets of Q and U maps, with corresponding vector catalogues, using ipoffset values of -0.3, 0 and +0.3. The visual differences between the vector maps should give a handle on the uncertainties caused by the IP model.

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