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Polarization Residuals

Massi presented a new method of correcting the phases due to polarization impurities in the feeds. The method was being tackled using data supplied by Foley. Another VLBI experiment might be scheduled in the near future for test purposes. Foley had contacted approx 100 PI's in attempt to obtain feedback for polarization cross talk at EVN antennas. He had only received approx 10 replies most of which suggested the level was around 3-5%, though it was much higher for Onsala (approx 20%). Schilizzi noted that this sets a limit for high dynamic range studies. Graham felt that the polarization residuals were much too high and noted that they were very sensitive to frequency. Changes across a 2 MHz band could be expected. Bregman explained that it was difficult to get it right even for a well understood system. Burgess was encouraged by Massi's analysis and was hopeful that with proper calibration the effect of polarization residuals could be calibrated out. Porcas noted that it would be more difficult to tackle amplitudes. Spencer felt that we should not rely totally on correction methods and that each station should be encouraged to eliminate residuals in hardware. Burgess noted that Jodrell's new (1990) 6 cm receiver had polarization cross-talk of < 3% (voltage).