Tests of the demodulating CCDs for the SPHERE / ZIMPOL imaging polarimeter

The imaging polarimeter ZIMPOL is one of three focal plane instruments of the SPHERE / VLT planet finder. ZIMPOL measures the linear polarization based on a fast modulation – demodulation principle using a charge-shifting technique on a masked CCD for separating the photons with opposite polarization direction. This paper describes the on-chip demodulation and the different detector read-out modes which are implemented for the ZIMPOL polarimeter. Test results are presented which allow an evaluation of the performance of the ZIMPOL CCD detectors. The achievable polarization efficiency is close to expectation and the charge trap correction with the two-phase demodulation mode works well. Other detector effects like bias level variations and read-out patterns can be corrected in the data reduction process. The tests demonstrate that the demodulating CCDs fulfill the requirements for the SPHERE project.