ESPRESSO - Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations

ESPRESSO The Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, is a super-stable Optical High Resolution Spectrograph for the combined coude' focus of the VLT. It can be operated by either one of the UTs or collecting the light from up to 4 UTs simultaneously.


Principal Investigator P.I. : Francesco Pepe (OG)
Co-I: Stefano Cristiani (INAF - Trieste), Rafael Rebolo (IAC), Nuno Santos (CAUP, U. do Porto)
Project Manager Denis Megevand (OG)

The ESPRESSO Consortium:

  • Observatoire de Geneva and University of Bern, Switzerland
  • INAF-Trieste and Brera, Italy
  • Instituto de Astrofisca de Canarias, Spain
  • Universidade do Porto and Lisboa, Portugal
  • ESO
ESO responsible / instrument scientist / project manager Luca Pasquini / Gaspare Lo Curto / Antonio Manescau
Project Status Preliminary Design Phase
Instrument Science Team E. Emsellem (ESO, Chair), C. Aerts (Instituut voor Sterrenkunde, Leuven, Belgium), M. Haehnelt (Institute of Astronomy, Cambridge, UK), A. Hatzes (Thringer Landessternwarte, Tautenburg, Germany), V. Hill (Observatoire de la Cote d'Azur, France), G. Tinetti (University College London, London, UK)
Location VLT combined coude' laboratory




  • Schedule: First light on telescope: goal 2016
  • Preliminary Acceptance Europe, October 2015
  • Final Design Review, April 2013
  • Preliminary Design Review, November 2011
  • Kick off Meeting, January 2011
  • Phase A Study Review Meeting, March 2010

Baseline Specification

Standard 1-UT
Very-High Res 1-UT
Wavelength Range
380-686 nm
380-686 nm
380-686 nm
Resolving Power
Aperture on Sky
1.0 arcsec
4x1.0 arcsec
0.5 arcsec
Sampling (average)
3.3 pixels
4.0 pixels (binned x2)
2.1 pixels
Spatial Sampling
6.9 pixels
4.0 pixels (binned x2)
3.5 pixels
Simultaneous reference
Yes (no sky)
Yes (no sky)
Yes (no sky)
Sky subtraction
Yes (no sim. ref.)
Yes (no sim. ref.)
Yes (no sim. ref.)
Total Efficiency
>10% at peak
>10% at peak
> 7% at peak
Instrumental RV precision (requirement)
<10 cm/sec
<=5 m/sec
<=5 m/sec

Scientific Objectives

The main scientific drivers for ESPRESSO are:

  • the measurement of high precision radial velocities of solar type stars for search for rocky planets
  • the measurement of the variation of the physical constants
  • the analysis of the chemical composition of stars in nearby galaxies

These science cases require an efficient, high-resolution, extremely stable and accurate spectrograph.

ESPRESSO, installed at the VLT facility, will combine unprecedented radial velocity precision with the large collecting area of the UTs. Moreover it will be capable of collecting the light simultaneously from the 4UTs, to measure precisely faint or high redshift objects.

The understanding of the formation and evolution of planetary systems is one of the most exciting science cases of these days. The radial velocity technique has been so far the most productive in terms of extra-solar planet detections. Low mass planets (one to few Earth masses) are especially interesting because according to formation models they could represent the bulk of the planet population. However they are more elusive and require extremely stable instruments. The HARPS instrument, with a precision better than 1m/s, has discovered up to now the vast majority of planets with masses smaller than Neptune, giving an invaluable experience in view of the realization of more precise instruments.
With a radial velocity precision better than 10cm/s, an Earth mass planet in the habitable zone of a low mass star can be detected.

The instrument will also have the capability to acquire the most accurate measurements of the fundamental constants α (the fine structure constant) and μ (the proton to electron mass ratio) as a function of redshift, therefore addressing the question of whether the constants of the Standard Model of Physics vary with the age of the Universe. Its capability to collect the light from all the 4 UTs simultaneously will enlarge the number of accessible QSOs with great benefit for this science case.

The combination of high resolution and high efficiency opens the possibility of measuring the chemical composition of stars in galaxies other than the Milky Way with unprecedented accuracy.

Given the large light collecting power and its efficiency, its high spectral resolution, and its extreme radial velocity precision and accuracy, we expect that ESPRESSO will not only fulfill its main scientific objectves, but also open new opportunities in observational Astronomy with hopefully new and unexpected results.

Instrument Description

ESPRESSO is a fiber-fed, cross-dispersed, high-resolution, echelle spectrograph. The telescope light is fed to the instrument via a Coude-Train optical system and fibers. ESPRESSO is located in the VLT Combined-Coude Laboratory (incoherent focus) where a front-end unit can combine the light from up to 4 Unit Telescopes (UT) of the VLT.


Fig. 1: ESPRESSO spectrograph concept at the Preliminary Design Review.

Instrument Design

The extreme precision will be obtained by adopting and improving well-known HARPS concepts. The light of 1 or several UTs is fed by means of the front-end unit into optical fibers which provide light scrambling and thus excellent stability of the spectrograph illumination. In order to improve light scrambling non-circular fiber shapes will be considered. (Laboratory tests on octagonal fibers have demonstrated that the scrambling efficiency can be increased by a factor of 10.) The target fiber can be fed either with the light of the astronomical object or the calibration sources. The reference fiber will receive either sky light (faint source mode) or calibration light (bright source mode). In the latter case - the simultaneous-reference technique adopted in HARPS - it will be possible to track instrumental drifts down to the cm/s level. This technique relies on the stability of the spectral lines of the calibration source. Therefore, ESPRESSO will integrate a new developed laser frequency comb which provides stability and accuracy of better than 10-11, in the mm/s regime.


Fig. 2: ESPRESSO spectrograph optical design at the Preliminary Design Review.

The target and sky light enter the instrument through two distinct optical fibers which form the slit of the spectrograph. Several optical tricks are used to obtain high spectral resolution and efficiency despite the large size of the telescope and the 1 arcscec fibre aperture. At the entrance of the spectrograph an anamorphic pupil slicing unit (APSU) shapes the beam in order to compress the beam in cross-dispersion direction but not in main-dispersion direction, where the full resolving power is required. In the latter direction, however, the pupil is sliced and superimposed on the echelle grating to minimize its size. The rectangular white pupil is then re-imaged and compressed. Given the wide spectral range and the high number of spectral channels, two large 90 mm x 90 mm CCDs are required to record the full spectrum. Therefore, a beam splitter separates the beam in a blue and a red arm which in turn allows to optimize each arm for image quality and optical efficiency. The cross-disperser has the function of separating the dispersed spectrum in all its spectral orders. In addition, an anamorphism is re-introduced to make the pupil square and to compress the order width in cross-dispersion direction, such that the inter-order space is maximized.

References and external links


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