On-sky performance and first results of the EXPRESSO instrument

Pepe et al report in their article, the EXPRESSO performances and the first results.

Abstract : ESPRESSO is the new high-resolution spectrograph of ESO’s Very-Large Telescope (VLT). It was designed for ultra-high radial-velocity precision and extreme spectral fidelity with the aim of performing exoplanet research and fundamental astrophysical experiments with unprecedented precision and accuracy. It is able to observe with any of the four Unit Telescopes (UT) of the VLT at a spectral resolving power of 140 000or 190 000 over the 378.2 to 788.7 nm wavelength range, or with all UTs together, turning the VLT into a 16-m diameter equivalent telescope interms of collecting area, while still providing a resolving power of 70 000.
Aim: We provide a general description of the ESPRESSO instrument, report on the actual on-sky performance, and present our Guaranteed-Time Observation (GTO) program with its first results.Methods.ESPRESSO was installed on the Paranal Observatory in fall 2017. Commissioning (on-sky testing) was conducted between December2017 and September 2018. The instrument saw its official start of operations on October 1st, 2018, but improvements to the instrument andre-commissioning runs were conducted until July 2019.
Results: The measured overall optical throughput of ESPRESSO at 550 nm and a seeing of 0.65′′exceeds the 10% mark under nominal astro-climatic conditions. We demonstrate a radial-velocity precision of better than 25 cm s−1during one night and 50 cm s−1over several months. These values being limited by photon noise and stellar jitter show that the performance is compatible with an instrumental precision of 10 cm s−1. No difference has been measured across the UTs neither in throughput nor RV precision.
Conclusions: The combination of the large collecting telescope area with the efficiency and the exquisite spectral fidelity of ESPRESSO opens a new parameter space in RV measurements, the study of planetary atmospheres, fundamental constants, stellar characterisation and many otherfields.