Polarimetric calibration results of an APERTIF phased array feed

The field-of-view of a reflector-based radio telescope can be significantly increased by using phased array feeds (PAFs). Such PAFs need to be polarimetrically calibrated to allow proper reconstruction of the polarization state of the radio waves received. In this paper, we present far field radiation patterns of the Aperture Tile-in-Focus (APERTIF) system on the Westerbork Synthesis Radio Telescope (WSRT) obtained for polarimetric characterization of the instrument. We also demonstrate successful application of these calibration results by reconstructing the rotation measure towards BL Lac over a frequency range from 1190 to 1390 MHz.