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. 
<br>
Values significantly different from zero might indicate
instrumental polarization. Since the error of the polarization plays
an important role here we plot it in the lower panel.
<br>
Only data with an error below 0.5 in instrumental polarization are trended.
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The corresponding instruments polarization normalized with its error used 
to be monitored. However, since 2024-04 we have been monitoring the actual 
value of the polarization.
<br>