Prospects and Needs of Micro-arcsecond Astrometry

Dedicated instruments for the VLTI, such as PRIMA [F. Delplancke, S.A. Leveque, P. Kervella, A. Glindemann, L. D’Arcio in Proceedings of the SPIE Conference: Astronomical Telescope and Instrumentation. Munich, Germany, 25–31 March 2000. Proc. SPIE, vol. 4006, p. 365; R. Launhardt et al. in Astrometry in the Age of the Next Generation of Large Telescopes. ASP Conf. Ser., vol. 338 (2005), p. 167] or the proposed second generation VLTI instrument GRAVITY [F. Eisenhauer, G. Perrin, S. Rabien, A. Eckart, P. Lena, R. Genzel, R. Abuter, T. Paumard in Astron. Nachr. 326, 561 (2005)], are designed for phase referenced imaging and aim in providing an astrometric accuracy of about 10 μas for two targets within the isoplanatic angle (PRIMA) or within the 2 arcsec FOV of the VLTI (GRAVITY). While interferometric measurements provide highly accurate astrometry, they are also time consuming and technically challenging. In the niche where precise but not necessarily accurate astrometry is needed, such as for measuring the reflex motion of stars orbited by extrasolar planets, single aperture measurements using AO imagers could provide an alternative. Providing astrometric precisions in the order of 100–200 μas, such measurements are feasible with current AO imagers and are a much easier and cheaper way to confirm and detect extrasolar planets. This article focuses on the possibilities and limitations of this technique, outlines the difference to interferometric measurements and shows its possible impact on current and future VLT instrumentation.