PRIMA is a facility that will enable astrometric observations with the Very Large Telescope Interferometer (VLTI). It aims at observing and characterising extra-solar planets through precise astrometry, which is a technique very complementary to the radial-velocity method. It has a different detection bias, favoring planets on wide orbits versus the short-period orbits preferentially detected by the radial-velocity technique. Moreover, astrometry measures two components (right ascension and declination) of the stellar reflex motion versus the single radial component that is observable spectroscopically.
The Geneva Observatory leads the ESPRI Consortium, which has contributed to the PRIMA project by providing the Differential Delay Lines (DDLs) and the PRIMA Astrometry Observing Software (PAOS). Currently ESO is testing and upgrading the PRIMA facility in order to enable astrometry at a level of 100 micro-arcseconds and better, a performance required for the exoplanet science case.