A cryogenic waveplate rotator for polarimetry at mm and submm wavelengths

Context. Polarimetry at mm and submm wavelengths is the new frontier of research in cosmic microwave background and interstellar dust studies. Polarimeters working in the IR to MM range need to be operated at cryogenic temperatures to limit the systematic effects related to the emission of the polarization analyzer. Aims. We study the effect of the temperature of the different components of a waveplate polarimeter and describe a system able to rotate a birefringent crystal at 4K in a completely automated way. Methods. We simulate the main systematic effects related to the temperature and non-ideality of the optical components in a Stokes polarimeter. To limit these effects, a cryogenic implementation of the polarimeter is mandatory. In our system, the rotation produced by a step motor running at room temperature is transmitted down to cryogenic temperatures by means of a long shaft and gears running on custom cryogenic bearings. Results. Our system is able to rotate a birefringent crystal at 4K in a completely automated way and dissipates only a few mW in the cold environment. A readout system based on optical fibers allows us to control the rotation of the crystal to better than 0.1 degrees. Conclusions. This device fulfills the stringent requirements for operations in cryogenic space experiments, such as the forthcoming PILOT, BOOMERanG and LSPE.