Summary:
New IP models are now available when reducing POL2 data. At 450 um the new model is a distinct improvement over the previous model. At 850 um the new and old models show little difference. To use the new models, it is necessary to add ipmodel=AUG2019 to the configuration when running pol2map:
% pol2map config='ipmodel=AUG2019'
Details:
Some POL2 users have seen evidence of significant inaccuracy in the model used to correct 450 um data for Instrumental Polarisation (IP). For instance, this can be seen in the following plots, which compare Q maps for DR21, created from observations taken in 2015 and 2017. The bottom left image shows the 450 um Q map from 2017 data, the bottom centre image shows the 450 um Q map from 2015 data and the bottom right image shows the difference. All these images use the same scaling. The difference appears to be a scaled version of the total intensity image (shown at top right), as confirmed by the scatter plot shown in the top centre.
The fact that the difference between the 2015 and 2017 Q maps looks like a scaled version of the total intensity map suggests strongly that the difference is caused by an inaccuracy in the IP correction (the IP correction subtracts a fraction of the total intensity map from the Q and U maps).
The current model used for IP correction is based on a set of observations of a bright unpolarised point source (Uranus). However, the high noise and low Q/U levels at 450 um in these Uranus observations makes it difficult to determine the model accurately. So a procedure has been developed that allows the IP model to be determined instead from a set of observations of any bright extended (possibly polarised) sources. This procedure has been applied to existing observations of OMC1/OrionA, DR21 and G034.257+0.155 to determine new IP models at both 850 um and 450 um. Observations of OrionB and Serpens main field2 were also used, but failed to produce any usable data at 450 um.
The new procedure is described in detail in the attached file.
Using the new 450 um IP model, there seems to be no significant difference between the Q/U maps made from the 2015 data and the 2017 data for DR21, as shown below:
and for completeness the corresponding U maps are shown below – again there is no significant difference between 2015 and 2017: