For polarized standard stars the pipeline determines the difference
between the measured angle and the angle tabulated in 
the catalogue. Since polarization parameters are usually tabulated for
broad-band filters the pipeline selects the broad-band filter closest to
the central wavelength of the grism. 
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Differences may indicate problems with the zero-points of the
retarder plate angle position.
Only data with an error of less than 10 degrees in angle offset are trended.