Solar polarimetry in the near UV with the Zurich Imaging Polarimeter ZIMPOL II

We describe an imaging polarimeter for high sensitivity measurements of solar polarisation signals in the wavelength range from 300 nm to 1 mum. At higher wavelengths the system is limited by the wavelength cut-off of the silicon CCD sensor used. To the blue the limitation arises from the atmospheric cut-off around 310 nm. The complete system is a modified version of the Zurich Imaging Polarimeter ZIMPOL II which has been equipped with a special CCD sensor. The CCD combines for the first time a so-called open electrode structure with on-chip demodulation. The concept as well as the detailed design of the instrument are presented. Examples of observations are shown and interpreted in order to experimentally evaluate the performance of the system.