ESPRESSO (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations) is a third-generation, fiber-fed, cross-dispersed, echelle spectrograph mounted on the European Southern Observatory's Very Large Telescope (VLT). The unit saw its first light with one VLT in December 2017 and first light with all four VLT units in February 2018.
ESPRESSO is the successor of a line of echelle spectrometers that include CORAVEL, Elodie, Coralie, and HARPS. It measures changes in the light spectrum with great sensitivity, and is being used to search for Earth-size rocky exoplanets via the radial velocity method. For example, Earth induces a radial-velocity variation of 9 cm/s on the Sun; this gravitational "wobble" causes minute variations in the color of sunlight, invisible to the human eye but detectable by the instrument. The telescope light is fed to the instrument, located in the VLT Combined-Coude Laboratory 70 meters away from the telescope, where the light from up to four unit telescopes of the VLT can be combined.
In 2026, a solar telescope was installed to observe our own Sun. The Paranal Solar ESPRESSO Telescope (PoET) uses a 60 cm mirror to collect light from the Sun, sending it to ESPRESSO for analysis. The telescope is capable of focusing on small features such as sun spots, and allows ESPRESSO to be used during the day, while continuing its VLT observations after sunset.
Contents
Sensitivity
ESPRESSO builds on the foundations laid by the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument at the 3.6-metre telescope at ESO's La Silla Observatory. ESPRESSO benefits not only from the much larger combined light-collecting capacity of the four 8.2-metre VLT Unit Telescopes, but also from improvements in the stability and calibration accuracy that are now possible by laser frequency comb technology. The requirement is to reach 10 cm/s, but the aimed goal is to obtain a precision level of a few cm/s. This would mean a large step forward over current radial-velocity spectrographs such as ESO's HARPS. The HARPS instrument can attain a precision of 97 cm/s (3.5 km/h), with an effective precision of the order of 30 cm/s. The ESPRESSO would greatly exceed this capability making detection of Earth-size planets from ground-based instruments possible. Commissioning of ESPRESSO at the VLT started late 2017.
The instrument is capable of operating in 1-UT mode (using one of the telescopes) and in 4-UT mode. In 4-UT mode, in which all the four 8-m telescopes are connected incoherently to form a 16-m equivalent telescope, the spectrograph detects extremely faint objects.
For example, for G2V type stars:
Rocky planets around stars as faint as V ≈ 9 (in 1-UT mode)
Neptune-mass planets around stars as faint as V ≈ 12 (in 4-UT mode )
Earth-size rocky planets around stars as faint as V ≈ 9 (CODEX on the E-ELT)
The best-suited candidate stars for ESPRESSO are non-active, non-rotating, quiet G dwarfs to red dwarfs. It operates at the peak of its efficiency for a spectral type up to M4-type stars.
Instrument
In the singleHR mode ESPRESSO can be fed by any of the four UTs.
Status
All design work was completed and finalised by April 2013, with the manufacturing phase of the project commencing thereafter. ESPRESSO was tested on June 3, 2016. ESPRESSO first light occurred on September 25, 2016, during which they spotted various objects, among them the star 60 Sgr A. After being shipped to Chile, installed at the VLT, ESPRESSO saw its first light there on 27 November 2017, in 1-UT mode, observing the star Tau Ceti; the first star observed in the 4-UT mode was on February 3, 2018.
A problem was discovered in the ESPRESSO charge-coupled device controllers, digital imaging hardware, where a differential nonlinearity issue has reduced the resolution obtainable more severely than was previously feared. The ESO detector team that determined the source of the problem is currently, as of June 2019, working on a new version of the associated hardware in order to remedy this hopefully temporary setback.
On August 29, 2019, the ESPRESSO ETC was updated to reflect the gain in transmission after the technical mission of July. This gain influx was, on average, ≈50% in the UHR and HR modes and ≈40% in the MR.
As of April 6, 2020, the red radial velocity detector has, at least for a very short time, achieved the ≈10 cm/s precision, while the blue detector has so far only managed ≈60 cm/s. Due to the limited spectral coverage and lack of reliability, the Laser Frequency Comb (LFC) is currently not integrated into the telescope and for now complete wavelength calibration will have to rely on the two backup ThAr lamps, with resultant radial velocity measurements values limited by photon noise, stellar jitter and so less precise than expected. The ESPRESSO operator and detector teams are working to characterize and correct the problem, with a dedicated mission expected to take place during 2020.
On May 24, 2020, a team led by A. Suárez Mascareño confirmed the existence of the exoplanet Proxima b, finding it to be about 1.17 times the mass of Earth—smaller than the older estimate of 1.3 times. They also suggested it is located in the habitable zone of its star, which it orbits in 11.2 days. ESPRESSO achieved an accuracy of 26 cm/s, about three times the accuracy obtained with HARPS. They also found a second signal in the data that could be of planetary origin with a semi-amplitude of only 40 cm/s and a 5.15-day period.
Scientific objectives
The main scientific objectives for ESPRESSO are:
The measurement of high precision radial velocities of solar type stars for the search for rocky planets in the habitable zone of their star.
The measurement of the variation of the physical constants
The analysis of the chemical composition of stars in nearby galaxies.
Consortium
ESPRESSO was developed by a consortium consisting on the European Southern Observatory (ESO) and seven scientific institutes:
Centre for Astrophysics of the University of Porto (Portugal)
Faculdade de Ciências da Universidade de Lisboa, CAAUL & LOLS (Portugal)
Trieste Astronomical Observatory (Italy)
Brera Astronomical Observatory (Italy)
Instituto de Astrofísica de Canarias (Spain)
Physics Institute of the University of Bern (Switzerland)
University of Geneva (Switzerland)
Institute of Astrophysics and Space Sciences (Portugal)
The principal investigator is Francesco Pepe.