Spectro-polarimetry of the standard star for linear polarization CD-28 13479 (=Hiltner 652) observed during the night 2009-04-04. In the PQ and PU panels, the blue solid lines represent the PQ and PU values after taking into account the combined correction due to the chromatism of the retarder waveplate and the Wollaston prism (i.e., wavelength dependent deviations of the position angle of the fast axis of the retarder waveplate and of the Wollaston prism from their respective nominal values), and the instrumental polarization; all discussed in the text. The error bars of each spectral point are represented in red. The PL panel shows the total fraction of linear polarization, and the bottom right panel shows the position angle of the observed polarization (blue solid lines). These observed values have to be corrected for the FORS2 instrument offset of the respective epoch to be fully consistent with their expected values, measured with FORS1. All dotted black lines show Stokes I expressed in arbitrary units.
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