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

Aim. We investigate the accuracy of astrometric measurements with the VLT/FORS1 camera and consider potential applications.

Methods. The study is based on two-epoch (2000 and 2002/2003) frame series of observations of a selected Galactic Bulge sky region that were obtained with FORS1 during four consecutive nights each. Reductions were carried out with a novel technique that eliminates atmospheric image motion and does not require a distinction between targets and reference objects.

Results. The positional astrometric precision was found to be limited only by the accuracy of the determination of the star photocentre, which is typically 200-300 mu as per single measurement for bright unsaturated stars B = 18-19. Several statistical tests showed that at time-scales of 1-4 nights the residual noise in measured positions is essentially a white noise with no systematic instrumental signature and no significant deviation from a Gaussian distribution. Some evidence of a good astrometric quality of the VLT for frames separated by two years was also found.

Conclusions. Our data show that the VLT with FORS1/2 cameras can be effectively used for astrometric observations of planetary microlensing events and other applications where a high accuracy is required, that is expected to reach 30-40 mu as for a series of 50 frames (one hour with R-special filter).