ESPRESSO at VLT

Context. ESPRESSO is the new high-resolution spectrograph of ESO’s Very Large Telescope (VLT). It was designed for ultra-high radial-velocity (RV) precision and extreme spectral fidelity with aim performing exoplanet research fundamental astrophysical experiments unprecedented accuracy. able to observe any four Unit Telescopes (UTs) VLT at a resolving power 140 000 or 190 over 378.2 788.7 nm wavelength range; it can also all UTs together, turning into 16 m diameter equivalent telescope in terms collecting area while still providing 70 000. Aims. We provide general description instrument, report on its on-sky performance, present our Guaranteed Time Observation (GTO) program along first results. Methods. installed Paranal Observatory fall 2017. Commissioning (on-sky testing) conducted between December 2017 September 2018. The instrument saw official start operations October 1, 2018, but improvements recommissioning runs were until July 2019. Results. measured overall optical throughput 550 seeing 0.65″ exceeds 10% mark under nominal astroclimatic conditions. demonstrate an RV better than 25 cm s −1 during single night 50 several months. These values being limited by photon noise stellar jitter shows that performance compatible instrumental 10 . No difference has been across UTs, neither nor precision. Conclusions. combination large efficiency exquisite opens parameter space measurements, study planetary atmospheres, constants, characterization, many other fields.