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We discuss a novel technique of determining the instrumental polarization parameters of a VLBI array, with the goal of imaging the linear polarization structure of compact radio sources. The source to be imaged is used in the calibration, avoiding the need for extensive observations of a separate source with known characteristics. The technique employs a non-iterative linear least-squares algorithm, and is based on the assumption that the linearly polarized emission has the same support as the total intensity emission (a physical necessity). This is a very strong constraint, since the parallactic angle rotation spreads the spurious emission due to uncorrected polarization leakage in the feeds unphysically over the polarization image. The technique is especially useful for the VLBA at its higher observing frequencies. It can also be used for polarization imaging of maser sources.
We have used this approach to image the polarization structure of the quasars 3C273, 3C345 and 3C279 at 22~GHz with the VLBA obtaining an angular resolution of ~250 microarcseconds. The images show interesting changes in the polarization degree and position angle over the jet. A possible interpretation of this behavior in the context of jet models is discussed.
