  VLBI observations are a reliable method to identify AGN, since they require
high brightness temperatures for a detection to be made. However, because of
the tiny fields of view it is unpractical to carry out VLBI observations of
many sources using conventional methods. We used an extension of the DiFX
software correlator to image with high sensitivity 96 sources in the Chandra
Deep Field South, using only 9h of observing time with the VLBA. We detected 20
sources, 8 of which had not been identified as AGN at any other wavelength,
despite the comprehensive coverage of this field. The lack of X-ray
counterparts to 1/3 of the VLBI-detected sources, despite the sensitivity of
co-located X-ray data, demonstrates that X-ray observations cannot be solely
relied upon when searching for AGN activity. Surprisingly, we find that sources
classified as type 1 QSOs using X-ray data are always detected, in contrast to
the 10% radio-loud objects which are found in optically-selected QSOs. We
present the continuation of this project with the goal to image 1450 sources in
the Lockman Hole/XMM region.
