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David Gordon
Type of contribution: oral

Revisiting the VLBA Calibrator Surveys, VCS-II

David Gordon, NVI Inc/NASA GSFC. VCS-II Team.

ICRF2 had 2 classes of sources. The first class was composed of sources observed in regular Mark3/Mark4/RDV VLBI geodesy and astrometry sessions over nearly 30 years. The second class was mostly composed of weaker sources observed in only one or a few specialized VLBA sessions called the 'VLBA Calibrator Surveys' (VCS), whose primary purpose was to vastly increase the pool of calibrator sources available for phase-referencing VLBI. The VCS sources composed 2/3 of the ICRF2 sources, but they had formal errors approximately 5 times greater, on average, than the other 1/3. In order to reduce this 2 class distinction, the VCS-II team was formed to re-observe these sources for ICRF3 and to improve their usefulness as phase-referencing calibrators. We report here on the re-observations of 2100 of these sources in 7 VLBA sessions in 2014 and 2015. The new observations were made at 2 GBits/sec versus 128 MBits/sec for the original VCS sessions, and thus are considerably more sensitive. Some 1813 'old' VCS sources have been re-observed with formal errors improving by an average factor of 3.6 in RA and Dec. And 274 'new' sources (not detected in the original VCS analysis) have now also been detected. Mapping of these sources is also being performed and we anticipate being able to compute structure indices for them as well.