2009A&A...498..591C

The study of the linear and circular polarization in AGN allows one to gain detailed information about the properties of the magnetic fields in these objects. However, the observation of circular polarization (CP) with single-dish radio telescopes, in particular, is usually difficult because of the expected weak signal. Measurements of CP are normally derived as (small) differences between two large numbers (left hand circular, LHC, and right hand circular, RHC), and an accurate calibration is absolutely necessary. We attempt to improve the calibration accuracy by including the Stokes parameter V in the common single-dish polarimetric measurements, thus allowing a full Stokes study of the source under examination. By completing a detailed study, up to 2nd order, of the Mueller matrix elements in terms of cross-talk components, we are able to reach the accuracy necessary to study circular polarization. The new calibration method was applied to data taken at the 100-m Effelsberg radio telescope during regular test observations of extragalactic sources at 2.8, 3.6, 6, and 11cm. The D-terms in phase and amplitude appear stable with time and the few known values of circular polarization were reproduced. Whenever a classical receiver and a multiplying polarimeter are available, we demonstrate that the proposed calibration scheme allows one to include Stokes V in standard single-dish polarimetric observations as the difference between two native circular outputs.