  In this paper we test the astrometric precision of VLT/FORS2 observations
using a serie of CCD frames taken in Galactic bulge area. A special reduction
method based on symmetrization of reference fields was used to reduce the
atmospheric image motion. Positional precision of unsaturated R=16 mag star
images at 17 sec exposure and 0.55 arcsec seeing was found to be equal to 300
microarcsec. The total error of observations was decomposed into components. It
was shown that astrometric error depends largely on the photon centroiding
error of the target (250 microarcsec for 16 mag stars) while the image motion
is much less (110 microarcsec). At galactic latitudes to about 20 degrees,
precision for a serie of frames with a 10 min total exposure is estimated to be
30-50 microarcsec for 14-16 mag stars providing the images are not overexposed
and the filter R "special" is used. Error estimates for fields with smaller sky
star density are given. We conclude that astrometric observations with large
telescopes, under optimal reduction, are never atmospheric limited. The bias
caused by differential chromatic refraction and residual chromatism of LADC is
considered and expressions valid for correcting color effects in the measured
positions are given.
