PRIMA, the instrument for Phase Referenced Imaging and Micro-arcsecond Astrometry, will implement the dual-feed capability at the ESO Very Large Telescope Interferometer (VLTI).

ESPRI will use PRIMA to combine the K-band light from two 1.8m diameter Auxiliary Telescopes (ATs) and interferometrically measure with very high accuracy the angular separation between two stars (astrometry). One of the two stars will typically be a distant background star (the reference star), the other one a nearby target star. From the relative motion of the two stars we will deduce the existence and orbital parameters of planets around the target star.

In order to reach this goal, a consortium lead by the Observatoire de Geneve (Switzerland), the Max Planck Institute for Astronomy, and the Landessternwarte Heidelberg, has built the Differential Delay Lines for PRIMA and develops the astrometric data reduction software and all necessary astrometric operation tools. The PRIMA facility is still under commissioning by ESO and is planned to become fully operational in 2014.

MPIA is involved with the science program, the development of the Differential Delay Line (DDL) hardware, the error budget, the development of the strategy and planning of observations and the data reduction software.

The ESPRI project has been terminated in 2014 because ESO has canceled the PRIMA project and thus the PRIMA facility will not be available for ground-based astrometry.
The ESPRI consortium will use the guaranteed observing time granted for it's contributions to PRIMA to carry out an alternative exoplanet search program via direct imaging with NaCo.
The new project web page can be found here.