A polarimeter with a new polarizer and a new device for eliminating the sky background polarization

The European Southern Observatory (ESO) has decided to procure a new two channel polarimeter which is a modification of the well known Serkowski-polarimeter (Frecker and Serkowski, 1976). The modification mainly consists of two parts. First, instead of a stationary superachromatic half-wave plate in front of the polarizing prism a pair of two phase plates with optical axes rotated by 45 deg against each other is located closely behind a double aperture diaphragm, thus allowing for an automatic elimination of the sky background polarization (Metz, 1977). Secondly, instead of a Wollaston prism a new type of polarizer will be used which combines the polarimetric accuracy of a Wollaston prism with the mechanical superiorities of a Foster prism.