Towards High-Precision Ground-Based Astrometry: Differential Delay Lines for PRIMA@VLTI

Deriving unambiguously the orbital parameters and masses of extrasolar planets requires at least 2-dimensional information on either the positions or motions of the planet directly (currently out of reach) or, indirectly, of the host star. The latter can be done with high-precision astrometry at the 10 microarcsec level. To achieve this goal, a consortium with partners from Germany, the Netherlands, and Switzerland, in agreement with ESO, will enhance the PRIMA system at the VLTI with Differential Delay Lines. The PRIMA facility will implement dual-star interferometry at the VLTI, thus enabling narrow-angle differential astrometry. The purpose of the Differential Delay Lines in PRIMA is to increase the astrometric accuracy by separating the large OPD correction terms which are common for target and reference star from the small differential terms, and to increase the sensitivity by stabilizing the fringe pattern and thus allow for longer integrations. This paper gives an overview of the PRIMA-DDL project, which consists of developing hardware, astrometric operation tools, and data reduction software, and outlines the anticipated astrometric planet search program to be carried out with this facility.