A serious bug in POL2MAP

A potentially serious bug has recently been found and fixed in the POL2MAP command.

Summary: If you run POL2MAP three times to produce a vector catalogue – once to create the auto-masked I map (step 1), once to create the externally-masked I map (step 2) and once to create the Q/U maps and vector catalogue (step 3) – and you set the BINSIZE parameter to a value larger than the PIXSIZE parameter (4 arc-seconds by default), then the I (total intensity) and P (polarisation) values in the final catalogue may be badly wrong (the angles in the vector catalogue are unaffected by this bug). But if you combine steps 2 and 3 into a single invocation of POL2MAP (as is done in the on-line POL2 tutorial), or if you do not set a value for the BINSIZE parameter, then all the values in the vector catalogue will be correct.

This bug has now been fixed and so it is safe to use the 3-step process with a large BINSIZE if you rsync starlink from EAO on 31st October or later. But existing vector catalogues created previously using  the 3-step process with a large BINSIZE should be treated with caution. As noted above, the I and P values in such catalogues could be badly wrong, but the ANG values will be correct. It may be advisable to re-create such catalogues using the new software and compare the old and new catalogues for differences. Note, since the I values may have changed, any vector selection criteria based on the I value or SNR may be affected.

Details: When the POL2MAP script needs to create an I, Q or U coadd map, it will re-use any previously created coadd if such a coadd exists with the correct name. This is what happens in the typical 3-step processing mode: step 3 re-uses the externally masked I coadd created at step 2. If the requested vector catalogue bin size (parameter BINSIZE) is set to a value larger than the map pixel size (parameter PIXSIZE), then the coadd maps are first binned up to the requested bin size before creating the catalogue. However, the bug described in this post resulted in this binning stage being skipped for any coadd that was created by a previous run of POL2MAP. So in a typical 3-step process (with BINSIZE set to, say, 12 arc-seconds) the Q and U coadds were correctly binned up to a pixel size of 12 arc-seconds at step 3 prior to creating the catalogue, but the pre-existing I coadd (created at step 2 with 4 arc-second pixels) was not binned up. This means that the I pixel value used to form each vector would come from the wrong point on the sky, resulting in both the I and the P (polarisation) value being wrong in the catalogue. Since the Q and U coadds are both created at the same time as the vector catalogue (i.e. in step 3) they would be binned correctly, and so the vector angles in the catalogue would be correct.

 

 

 

 

 

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