High dynamic range imaging with the EVN

It has been claimed that MkIII/EVN experiments do not achieve the expected level of image quality, or dynamic range. An EVN test observation of a strong and compact source, DA193, has allowed us to investigate in detail the possible systematic errors affecting EVN images. Indeed, we have identified two effects which both significantly affect image quality: instrumental polarization and an amplitude error caused by the stepwise delay tracking in XF-correlators. This paper shows how these effects influence total-intensity images and discusses the correction of these non-closing errors. Their removal increased the peak/rms dynamic range from 5000 to 19,000 in our image of DA193.