Observations with the European VLBI-Network at 6 cm have shown that the standard deviation of the closure phase on a significant sample of measurements, is almost one order of magnitude higher than expected from the associated thermal noise (Massi et al, 1996). This fact is a clear indication that there are still systematic errors in the experimental technique and/or data reduction procedures. The consequence is a significant degradation of the quality of the final image: theoretically detectable weak features are lost because of the increased "noise" level. Here we demonstrate that the instrumental polarization is responsible for these residual errors in observations with the European VLBI Network.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998