2017MNRAS.464.4146C

We reduced European Southern Observatory's archival linear spectropolarimetry data (4000-9000 Å) of six highly polarized and eight unpolarized standard stars observed between 2010 and 2016, for a total of 70 epochs, with the FOcal Reducer and low dispersion Spectrograph (FORS2) mounted at the Very Large Telescope. We provide very accurate standard stars polarization measurements as a function of wavelength, and test the performance of the spectropolarimetric mode (PMOS) of FORS2. We used the unpolarized stars to test the time stability of the PMOS mode, and found a small (<=0.1 per cent), but statistically significant, on-axis instrumental polarization wavelength dependence, possibly caused by the tilted surfaces of the dispersive element. The polarization degree and angle are found to be stable at the level of <=0.1 per cent and <=0.2 degrees, respectively. We derived the polarization wavelength dependence of the polarized standard stars and found that, in general, the results are consistent with those reported in the literature, e.g. Fossati et al., who performed a similar analysis using FORS1 data. The re-calibrated data provide a very accurate set of standards, which can be very reliably used for technical and scientific purposes. The analysis of the Serkowski parameters revealed a systematic deviation from the width parameter K reported by Whittet et al. This is most likely explained by incorrect effective wavelengths adopted in that study for the R and I bands.