Foreground: S-PASS 2.3 GHz
You are welcome to use the S-PASS data, but the S-PASS team requests that:
S-PASS follows the IAU convention on Polarisation Angles (PA), as is used for most astrophysical data.
That means PA is 0 northward (in the coordinate system used) and increases eastward.
However, there are a few data sets and software packages that use a different convention, with PA increasing westward. Examples are WMAP and Planck data, and the HEALPix software package. Passing from one system to the other means simply changing the sign of Stokes U (or the sign of the PA). For more information, see the LAMBDA Polarization Convention page.
To reflect this the S-PASS HEALPix fits file headers contain the keyword POLCCONV = 'IAU'.
The user should bear this in mind when using S-PASS (and most astrophysical data sets) in combination with WMAP and Planck data (and possibly other CMB data sets) or with the HEALPix package.
Recommended download procedure for '.fits' or '.tar.gz' files: