W. Alef, A. Benz, J. Conway, M. Pestalozzi, T. Beasley
Phase-reference mapping of nearby stars: status report
 
                   ABSTRACT
 
 
We have observed radio emitting stars in  phase-referenced mode with both
the VLBA and the  EVN in order  to map their brightness distributions and
to determine precise astrometric  parameters.  While the VLBA  correlator
uses the CALC  8 model, the older  MK3 correlator model is not sufficient
to make phase-referenced mapping possible  using the visibility amplitude
and residual phase.  In the raw MK3 correlated  data we have successfully
replaced the correlator model with the CALC 8 model.   We have shown that
phase-reference mapping is possible with MK3  data for source separations
of  2 to  3 degrees.   At present  we  are exploring  the possibility  of
removing   ionospheric  and   atmospheric  contributions, which  are  not
modelled at all in CALC 8, and which decrease  the SNR in phase-reference
maps in general by a factor of 2 to 3.
 
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In Proceedings of the 13th Working Meeting on European VLBI for Geodesy
and Astrometry, Viechtach, 1999
 
Keywords: VLBI, phase-referencing