Full Stokes polarimetric observations with a single-dish radio-telescope
  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, especially the observation of circular polarization (CP) with
single-dish radio-telescopes is usually difficult because of the weak signals
to be expected. Normally CP is derived as the (small) difference of two large
numbers (LHC and RHC); hence an accurate calibration is absolutely necessary.
Our aim is to improve the calibration accuracy to include the Stokes parameter
V in the common single-dish polarimetric measurements, allowing a full Stokes
study of the source under examination. A detailed study, up to the 2nd order,
of the Mueller matrix elements in terms of cross-talk components allows us to
reach the accuracy necessary to study circular polarization. The new
calibration method has been 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 11 cm. The D-terms in phase and amplitude appear very stable
with time and the few known values of circular polarization have been
confirmed. It is shown that, whenever a classical receiver and a multiplying
polarimeter are available, the proposed calibration scheme allows one to
include Stokes V in standard single-dish polarimetric observations as
difference of two native circular outputs.
