A high-resolution, ultra stable, fiber-fed cross-dispersed echelle spectrograph, called ESPRESSO, is the successor of a line of echelle spectrometers (CORAVEL, Elodie, Coralie, HARPS) designed to give high accuracy radial-velocity (RV) measurements. It combines the best properties of two leading instruments, HARPS and UVES, enhancing the stability of HARPS with an unprecedented resolution.

ESPRESSO stands for Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations. This instrument is intended as an additional second-generation instrument for the VLT, as recommended by the ESO STC during its 67th meeting, in October 2007 and the ESO council at its 11th meeting in December 2007.

The main science driver will be the detection and characterization of Earth twins planets in the habitable zone, but many additional science cases will benefit from its highly stable spectroscopic observations.

The facility will be installed in the underground Combined Coudé Laboratory (CCL) of the VLT, and will be linked to the four UT telescopes through four optical paths called Coudé Trains. Thanks to these optical links and to its special Front-End (F/E) subsystem, it will be able to collect the light of any UT independently or together, allowing several different configurations amongst which a 4-UT mode which will make it the first instrument mounted on a 16-m equivalent telescope, mode originally foreseen at the VLT. This linking scheme presents several advantages, but also makes the interface with the VLT more complex than for an instrument fed by a single telescope. It impacts on the complexity of the relationship between the consortium providing the instrument and ESO, the customer.

The targeted high RV accuracy requires very high performances in stability and resolution, which in turn require adequate technical solutions at several levels.

The ESPRESSO consortium is composed of Italian, Portuguese, Spanish and Swiss institutes. ESO is not properly member of the consortium, but participates in the project as associated partner.

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