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We investigate the accuracy of astrometric measurements with the VLT/FORS1camera and consider potential applications. The study is based on two-epoch(2000 and 2002/2003) frame series of observations of a selected Galactic Bulgesky region that were obtained with FORS1 during four consecutive nights each.Reductions were carried out with a novel technique that eliminates atmosphericimage motion and does not require a distinction between targets and referenceobjects. The positional astrometric precision was found to be limited only bythe accuracy of the determination of the star photocentre, which is typically200-300 microarcsec per single measurement for bright unsaturated starsB=18-19. Several statistical tests have shown that at time-scales of 1-4 nightsthe residual noise in measured positions is essentially a white noise with nosystematic instrumental signature and no significant deviation from a Gaussiandistribution. Some evidence of a good astrometric quality of the VLT for framesseparated by two years has also been found. Our data show that the VLT withFORS1/2 cameras can be effectively used for astrometric observations ofplanetary microlensing events and other applications where a high accuracy isrequired, that is expected to reach 30-40 microarcsec for a series of 50 frames(one hours with R filter).

High-precision astrometry on the VLT/FORS1 at time scales of few days