  The apparent position of the origin (core) of extragalactic radio jets shifts
with the observing frequency, owing to synchrotron self-absorption and external
absorption. One of the largest core shifts was detected by us in the quasar
0850+581 between 2 and 8 GHz. We have followed this up recently by a dedicated
VLBA experiment at 5, 8, 15, 24, and 43 GHz. First results from this study
enabled estimating the absolute geometry and physical conditions in the
parsec-scale jet origin.
