Systematics of Microwave Polarimetry with the Planck LFI

Abstract. The Planck Low Frequency Instrument will recover polarization by differencing the outputs from radiometers sensitive to orthogonal polarizations. We contrast the systematic errors that afflict such a system with those that affect correlation polarimeters; the Planck design has some important advantages when measuring very weak signals such as the CMB. We also review systematic effects arising from the choice of scan strategy for all-sky mapping missions like Planck. DIFFERENCING VS. CORRELATION POLARIMETERS Radiometers can be used to measure polarization in two fundamentally different ways. Differencing polarimeters work in much the same way as polarimeters used in other wavebands; that is, the radiation is filtered to isolate components in several different pure polarization states and these are combined to derive the required Stokes parameters. The Planck Low Frequency Instrument (LFI), described in these proceedings by Villa, is of this type. Each feed horn couples the incoming radiation to a waveguide, essentially preserving the polarization state (deviations from this approximation will be discussed later). An OMT separates the radiation into two orthogonal linear componenents (‘X ’ and ‘Y’) which are separately amplified and square-law detected. Ideally, the sum of these signals is proportional to Stokes I, while the difference measures one component of the (Q,U) vector. This is formally equivalent to a Wollaston prism system in optical polarimetry. To obtain the full linear polarization vector, more