[Paper Review] ESPRESSO at VLT. On-sky performance and first results

F. Pepe, S. Cristiani+98 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|Oct 1, 2020

Stellar, planetary, and galactic studies97 references51 citations

TL;DR

ESPRESSO describes the VLT ultra-high-resolution spectrograph, its commissioning on-sky performance, and first guaranteed-time results achieving high radial-velocity precision.

ABSTRACT

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 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 (UTs) of the VLT at a spectral resolving power of 140 000 or 190 000 over the 378.2 to 788.7 nm wavelength range; it can also observe with all four UTs together, turning the VLT into a 16 m diameter equivalent telescope in terms of collecting area while still providing a resolving power of 70 000. Aims. We provide a general description of the ESPRESSO instrument, report on its on-sky performance, and present our Guaranteed Time Observation (GTO) program along with its first results. Methods. ESPRESSO was installed on the Paranal Observatory in fall 2017. Commissioning (on-sky testing) was conducted between December 2017 and September 2018. The instrument saw its official start of operations on October 1, 2018, but improvements to the instrument and recommissioning 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 astroclimatic conditions. We demonstrate an RV precision of better than 25 cm s−1 during a single night and 50 cm s−1 over several months. These values being limited by photon noise and stellar jitter shows 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 characterization, and many other fields.

Motivation & Objective

• Describe the ESPRESSO instrument and subsystems and their integration at the VLT.
• Report the on-sky performance and commissioning progress up to start of operations.
• Present the Guaranteed-Time Observation (GTO) program and initial results.
• Assess optical throughput, radial-velocity precision, and instrumental stability.
• Discuss implications for exoplanet science, fundamental constants, and stellar characterization.

Proposed method

• Commissioning and on-sky testing of ESPRESSO from 2017 to 2019 to validate performance.
• Fiber-fed, cross-dispersed, high-resolution echelle spectrograph design feeding 1-UT and 4-UT configurations.
• Simultaneous drift measurement using a second fiber for wavelength calibration (simultaneous-reference mode).
• Wavelength calibration with ThAr lamp plus Fabry-Pérot; Laser Frequency Comb (LFC) available but not yet integrated.
• Throughput, RV precision, and stability analyses under nominal astro-climatic conditions; consideration of photon noise and stellar jitter as limiting factors.
• Addressing 1-UT vs 4-UT performance and implementing the HR42 mode to extend magnitude range.

Experimental results

Research questions

• RQ1 What is the on-sky performance and throughput of ESPRESSO under nominal conditions?
• RQ2 Can ESPRESSO achieve radial-velocity precision at or near 10 cm s-1 instrumental limit, and what limits remain (photon noise, stellar jitter)?
• RQ3 How do the 1-UT and 4-UT configurations compare in terms of throughput and RV precision?
• RQ4 What first science results can be obtained from ESPRESSO's Guaranteed-Time Observations (GTO) program?

Key findings

• Measured overall optical throughput at 550 nm with 0.65″ seeing exceeds 10% under nominal conditions.
• Radial-velocity precision better than 25 cm s−1 during one night and 50 cm s−1 over several months.
• Performance limited by photon noise and stellar jitter, indicating compatibility with an instrumental precision of 10 cm s−1.
• No measured difference across the Unit Telescopes in throughput or RV precision.
• HR42 mode was introduced to push the read-out-noise-limited magnitude range to fainter magnitudes.