SPHERE / ZIMPOL: characterization of the FLC polarization modulator

ZIMPOL is an imaging polarimeter for the high-contrast SPHERE/VLT "planet finder" instrument using fast polarization modulation and demodulating CCD detectors. The polarimetric performance of the ZIMPOL instrument depends on the polarimetric alignment and quality of the polarization components. This paper gives an overview on the polarimetric concept and the calibration plan of ZIMPOL. We discuss in particular the alignment of the polarimetric calibration components and the polarimetric properties of the ferro-electric liquid crystal (FLC) modulator package used in ZIMPOL. Our measurements demonstrate the good broad-band performance of the modulator. Faint targets, like extra-solar planets, require mainly a high polarimetric efficiency while for detailed studies of bright targets a good characterization of the modulator package is essential. Therefore we quantify in detail the wavelength dependence of the polarimetric efficiency and the cross-talk effects which have to be taken into account in the calibration and data reduction process of high S/N measurements.