SIMBAD references

This paper belongs to a series of articles which aim at determining higher quality maps from VLBI measurements. We discuss the result of a statistical study on closure phases of point-like sources observed with the EVN at 6 cm. Such a quantity is a combination of baseline based errors and thermal noise. The average closure phase is found to be in the range 0.2deg -0.5deg while the associated thermal noise contribution is found to be almost one order of magnitude lower. In order to quantify this effect in terms of dynamic range degradation we have added such closure errors to simulated data of an EVN observation of a point source. The dynamic range ratio between two maps, one produced with thermal noise only, and the other with closure errors is a factor of 6. In the second part of this paper is discussed how the instrumental polarization contributes significantly to the closure phase. Its removal should therefore improve the quality of the maps. Finally problems connected with the cross polarization data for baselines with Westerbork along with the necessity of a similar analysis to be performed in each 2 MHz frequency channel are discussed.