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ESPRESSO spectrograph concept at the Preliminary Design Review. ESPRESSO spectrograph concept at the Preliminary Design Review..jpg
ESPRESSO spectrograph concept at the Preliminary Design Review.
ESPRESSO spectrograph optical design at the Preliminary Design Review. ESPRESSO spectrograph optical design at the Preliminary Design Review..jpg
ESPRESSO spectrograph optical design at the Preliminary Design Review.
ESPRESSO successfully made its first observations in November 2017.

ESPRESSO (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations) [1] 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 on September 25, 2016. [2] [3]

Optical fiber light-conducting fiber

An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer excessively. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers.

Echelle grating Type of diffraction grating used in spectrometers

An echelle grating is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at high incidence angles and therefore in high diffraction orders. Higher diffraction orders allow for increased dispersion (spacing) of spectral features at the detector, enabling increased differentiation of these features. Echelle gratings are, like other types of diffraction gratings, used in spectrometers and similar instruments. They are most useful in cross-dispersed high resolution spectrographs, such as HARPS, PRL Advanced Radial Velocity Abu Sky Search (PARAS), and numerous other astronomical instruments.

European Southern Observatory intergovernmental research organization for ground-based astronomy

The European Southern Observatory (ESO), formally the European Organisation for Astronomical Research in the Southern Hemisphere, is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky. The organisation employs about 730 staff members and receives annual member state contributions of approximately €162 million. Its observatories are located in northern Chile.


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 will be 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. [4] 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. The Principal Investigator is Francesco Pepe.

High Accuracy Radial Velocity Planet Searcher high-precision echelle spectrograph

The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. HARPS has discovered over 130 exoplanets to date, with the first one in 2004, making it the most successful planet finder behind the Kepler space observatory. It is a second-generation radial-velocity spectrograph, based on experience with the ELODIE and CORALIE instruments.

The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies.

Terrestrial planet planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars

A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, "Earth-like". These planets are located between the Sun and the Asteroid Belt.


Data from ESPRESSO First Light. ESPRESSO first light spectrum.jpg
Data from ESPRESSO First Light.

ESPRESSO will build 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 will benefit 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, [6] 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 like ESO's HARPS. The HARPS instrument can attain a precision of 97 cm/s (3.5 km/h), [7] with an effective precision of the order of 30 cm/s, [8] making it one of only two spectrographs worldwide with such accuracy.[ citation needed ] 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.

La Silla Observatory astronomical observatory in Chile

La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory (ESO). Several other telescopes are located at the site and are partly maintained by ESO. The observatory is one of the largest in the Southern Hemisphere and was the first in Chile to be used by ESO.

Frequency comb laser source emitting in equally spaced frequency lines

In optics, a frequency comb is a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines. Frequency combs can be generated by a number of mechanisms, including periodic modulation of a continuous-wave laser, four-wave mixing in nonlinear media, or stabilization of the pulse train generated by a mode-locked laser. Much work has been devoted to the latter mechanism, which was developed around the turn of the 21st century and ultimately led to one half of the Nobel Prize in Physics being shared by John L. Hall and Theodor W. Hänsch in 2005.

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 will detect extremely faint objects. [4] [9]

For example, for G2V type stars:

ESPRESSO will focus the observations on the best-suited candidates: non-active, non-rotating, quiet G dwarfs to red dwarfs. It will operate at the peak of its efficiency for a spectral type up to M4-type stars.

G-type main-sequence star main-sequence star

A G-type main-sequence star, often called a yellow dwarf, or G dwarf star, is a main-sequence star of spectral type G. Such a star has about 0.84 to 1.15 solar masses and surface temperature of between 5,300 and 6,000 K., Tables VII, VIII. Like other main-sequence stars, a G-type main-sequence star is converting the element hydrogen to helium in its core by means of nuclear fusion. The Sun, the star to which the Earth is gravitationally bound in the Solar System and the object with the largest apparent magnitude, is an example of a G-type main-sequence star. Each second, the Sun fuses approximately 600 million tons of hydrogen to helium, converting about 4 million tons of matter to energy. Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri A, Tau Ceti, and 51 Pegasi.

Red dwarf An informal category of small, cool stars on the main sequence

A red dwarf is a small and cool star on the main sequence, of M spectral type. Red dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included.


First light of the ESPRESSO instrument with all four unit telescopes ESPRESSO instrument achieves first light with all four Unit Telescopes.jpg
First light of the ESPRESSO instrument with all four unit telescopes

ESPRESSO will use as calibration a laser frequency comb (LFC), with backup of two Th Ar lamps. It will have three instrumental modes: singleHR, singleUHR and multiMR. In the singleHR mode ESPRESSO can be fed by any of the four UTs. [12]


Engineering rendering of the ESPRESSO instrument Engineering rendering of the ESPRESSO instrument.jpg
Engineering rendering of the ESPRESSO instrument

All design work was completed and finalised by April 2013, with the manufacturing phase of the project commencing thereafter. [1] ESPRESSO was tested on June 3, 2016. [14] ESPRESSO first light occurred on September 25, 2016, during which they spotted various objects, among them the star 60 Sgr A. [2] [3] 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; [15] [16] [17] the first star observed in the 4-UT mode was on February 3, 2018. [18] [19] [20]

ESPRESSO in the 1-UT mode (one single telescope used) has been opened to the astronomical community and producing scientific data since October 24, 2018. On quiet stars it has already demonstrated radial-velocity precision of 25 cm/s over a full night. However, there have been some teething problems, for example, in light collecting efficiency which was around 30% lower than expected and required. And so, some fine tuning, including replacing the parts causing the efficiency problem and subsequent re-testing, will be made on the instrument before the full 4-UT mode is open to the scientific community in April 2019. [21]

Scientific objectives

The main scientific objectives for ESPRESSO are: [22] [23]


ESPRESSO is being developed by a consortium consisting on the European Southern Observatory (ESO) and seven scientific institutes:

ESPRESSO specifications

Telescope VLT (8m)
ScopeRocky planets
Sky aperture 4 arcsec
λ coverage380 nm-686  nm [24]
λ precision5  m/s
RV stability< 10 cm/s
4-VLT mode (D = 16 m) with RV = 1 m/s
Source: [10] [25] [23]

Radial velocity comparison tables

Planet Mass Distance
Radial velocity
Jupiter 128.4 m/s
Jupiter 512.7 m/s
Neptune 0.14.8 m/s
Neptune 11.5 m/s
Super-Earth (5 M⊕)0.11.4 m/s
Alpha Centauri Bb (1.13 ± 0.09 M⊕)0.040.51 m/s(1 [26] )
Super-Earth (5 M⊕)10.45 m/s
Earth 0.090.30 m/s
Earth 10.09 m/s
Source: Luca Pasquini, power-point presentation, 2009 [10] Notes: (1) Most precise vradial measurements ever recorded. ESO's HARPS spectrograph was used. [26] [27]
Planets [10]
PlanetPlanet Type
Semimajor Axis
Orbital Period
Radial velocity
Detectable by:
51 Pegasi b Hot Jupiter 0.054.23 days55.9 [28] First-generation spectrograph
55 Cancri d Gas giant 5.7714.29 years45.2 [29] First-generation spectrograph
Jupiter Gas giant 5.2011.86 years12.4 [30] First-generation spectrograph
Gliese 581c Super-Earth 0.0712.92 days3.18 [31] Second-generation spectrograph
Saturn Gas giant 9.5829.46 years2.75Second-generation spectrograph
Proxima Centauri b Habitable planet (potentially) 0.0511.19 days1.38 [32] Second-generation spectrograph
Alpha Centauri Bb Terrestrial planet 0.043.23 days0.510 [33] Second-generation spectrograph
Neptune Ice giant 30.10164.79 years0.281Third-generation spectrograph
Earth Habitable planet 1.00365.26 days0.089Third-generation spectrograph (likely)
Pluto Dwarf planet 39.26246.04 years0.00003Not detectable

MK-type stars with planets in the habitable zone

Stellar mass
Planetary mass
Source: [34] [35]

See also

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