4. Comparison between Observations and Model
As stated in Sect. 2.2, in absence of errors, the closure phase of a point source should differ from zero only due to noise contributions. In Figure 1 the closure phase is given for the parallel-hand data (LL) of three triangles. The data are not noise-limited, on the contrary a regular trend is there evident, which proves that a systematic error still affects the data. This effect has not been noticed before because the closure phase of a point like source was never up to now monitored with a proper scale, for such long period and at such time resolution as in our "ad hoc" observation.
The contribution of the instrumental polarization to the closure phase, computed by using equation (10) and the D terms derived with the Brandeis software, is also plotted in Fig.1. As one can see equation 10 reproduces completely the observed trend. The agreement between data (closure phase) and equation 10 (contribution of the instrumental polarization to the closure phase) proves that the systematic errors present up to now in European VLBI data is in fact due to the high instrumental polarization of some of the telescopes.
The effects of its removal in terms of improvement of the dynamic range will be shown in the next paper (Paper IV). Preliminary tests show an improvement in dynamic range from 5000 to 9000. A work is in progress to optimize the D terms delivered by AIPS by fitting equation 10 to the closure phase of a point like source.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998