For unpolarized standard stars the pipeline determines the measured
value. 
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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Values significantly different from zero might indicate
instrumental polarization. Since the error of the polarization plays
an important role here we plot the instruments polarization normalized
with its error.

Only data with an error below 0.5 in instrumental polarization are trended.