RISTRETTO: a pathfinder instrument for exoplanet atmosphere characterization

We introduce the RISTRETTO instrument for ESO VLT, an evolution from the original idea of connecting the SPHERE high-contrast facility to the ESPRESSO spectrograph(1). RISTRETTO is an independent, AO-fed spectrograph proposed as a visitor instrument, with the goal of detecting and characterizing nearby exoplanets in reflected light for the first time. RISTRETTO aims at characterizing the atmospheres of Proxima b and several other exoplanets using the technique of high-contrast, high-resolution spectroscopy. The instrument is composed of two parts: a front-end to be installed on VLT UT4 providing a two-stage adaptive optics system, using the AOF facility as a first stage, with coronagraphic capability and a 7-fiber IFU, and a diffraction-limited R=135,000 spectrograph in the 620-840 run range. We present the requirements and the preliminary design of the instrument.