  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%.
