2011A&A...529A..57P

We present a full analysis of the CAFOS polarimeter mounted at the Calar Alto 2.2m telescope. This provides future users of this mode with all necessary information to properly correct for instrumental effects in polarization data obtained with this instrument. The standard stars BD+59d389 (polarized) and HD 14069 (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. CAFOS shows a roughly constant instrumental polarization at the level of ∼0.3% between 4000Å and 8600Å. Below 4000Å, the spurious polarization grows to reach ∼0.7% at 3600Å. This instrumental effect is most likely produced by the telescope optics, and appears to be additive. The Wollaston prism clearly operates less than ideally. However, the problem is largely removed by using at least four 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 7400Å. After correcting for instrumental polarization and retarder plate chromatism, with four half-wave plate angles CAFOS can reach an rms linear polarization accuracy of about 0.1%.