Characterisation of the CAFOS linear spectro-polarimeter
  Aims. This research note presents a full analysis of the CAFOS polarimeter mounted at the Calar Alto 2.2m telescope. It also provides future users of this mode with all necessary information to properly correct for instrumental effects in polarization data obtained with this instrument.   Methods. The standard stars BD+59d389 (polarized) and HD14069 (unpolarized) were observed with CAFOS in November, 2010, using 16 half-wave plate angles. The linear spectropolarimetric properties of CAFOS were studied using a Fourier Analysis of the resulting data.   Results. CAFOS shows a roughly constant instrumental polarization at the level of ~0.3% between 4000 and 8600 A. Below 4000 A the spurious polarization grows to reach ~0.7% at 3600 A. This instrumental effect is most likely produced by the telescope optics, and appears to be additive. The Wollaston prism presents a clear deviation from the ideal behavior. The problem is largely removed by the usage of at least 4 retarder plate angles. The chromatism of the half-wave plate causes a peak-to-peak oscillation of ~11 degrees in the polarization angle. This can be effectively corrected using the tabulated values presented in this paper. The Fourier analysis shows that the k!=0,4 harmonics are practically negligible between 3800 and 7400 A.   Conclusions. After correcting for instrumental polarization and retarder plate chromatism, with 4 half-wave plate angles CAFOS can reach an rms linear polarization accuracy of about 0.1%. 
F. Patat and S. Taubenberger
astro-ph.IM