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XPOL, the first correlation polarimeter at a large millimeter telescope, usesa flexible digital correlator to measure all four Stokes parameterssimultaneously, i.e. the total power I, the linear polarization components Qand U, and the circular polarization V. The versatility of the backend providesadequate bandwidth for efficient continuum observations as well as sufficientspectral resolution (40 kHz) for observations of narrow lines. We demonstratethat the polarimetry specific calibrations are handled with sufficientprecision, in particular the relative phase between the Observatory's twoorthogonally linearly polarized receivers. The many facets of instrumentalpolarization are studied at 3mm wavelength in all Stokes parameters: on-axiswith point sources and off-axis with beam maps. Stokes Q which is measured asthe power difference between the receivers is affected by instrumentalpolarization at the 1.5% level. Stokes U and V which are measured as crosscorrelations are very little affected (maximum sidelobes 0.6% (U) and 0.3%(V)). These levels critically depend on the precision of the receiveralignment. They reach these minimum levels set by small ellipticities of thefeed horns when alignment is optimum (<~ 0.3"). A second critical prerequisitefor low polarization sidelobes turned out to be the correct orientation of thepolarization splitter grid. Its cross polarization properties are modeled indetail. XPOL observations are therefore limited only by receiver noise inStokes U and V even for extended sources. Systematic effects set in at the 1.5%level in observations of Stokes Q. With proper precautions, this limitation canbe overcome for point sources. Stokes Q observations of extended sources arethe most difficult with XPOL.

XPOL - the correlation polarimeter at the IRAM 30m telescope