Polarimetry developments at the Gemini telescopes

Gemini polarimetry is based on a waveplate module located in the base of the A&G unit that can be used by all instruments operating in the optical and near-infrared. Because of space limitations and limited access to the module, a single half-wave retarder covering 0.34 to 2.5μm is used for linear polarimetry. A composite zero-order half-wave retarder is used for the L-band (mid-IR instruments have their own waveplate module). The plates have a clear aperture of 95mm and are surrounded by a transparent annulus to increase the field of view for the on-instrument wavefront sensors. Each instrument includes, or will include, a 2-beam polarising prism, usually in the form of a Wollaston prism. Provision for circular polarimetry has been included but not yet implemented. The design of the waveplate module and the techniques employed to provide high precision are described. The materials available for the Wollaston prisms, including those used in the mid-IR, are also discussed. Techniques to avoid ripple in the polarisation spectrum observed with some spectrometers are presented. Unfortunately at present observational results are not available to include in this paper.