ESPRESSO: The VLT planet hunter


ESPRESSO is the ESO/VLT high-resolution spectrograph for measuring precise radial velocities on a long timespan with the main scientific aim to detect and characterise Earth twins in the inhabitable zone of solar-like stars.

ESPRESSO is a highly-stabilized fibre-fed échelle spectrograph that can be fed with light from either one or the four Unit Telescopes of the VLT. The instrument is installed at the incoherent combined Coudé focus (ICCF) of the VLT. The light from the astronomical source is redirected from the telescopes to the detectors through three components of the ICCF facility: the Coudé trains, the front ends, and the spectrograph. The Coudé Trains (CT) bring the light from each telescope to the Combined Coudé Lab (CCL) through 13 optical elements, including mirrors, lenses, and prisms. The four Front Ends (one for each UT) receive the light from the CTs and feed the entrance fibres. The Fibre Link transports the light from the Front Ends to the vacuum vessel. The latter is thermally stabilized at the mK level. The light is then going through the different optical elements of the spectrograph and splitted up into a red and a blue spectrum recorded on the corresponding red and blue cameras.

The spectrograph is fed by two fibres, one for the target itself and the other for simultaneous calibration (either sky or simultaneous reference: Laser Frequency Comb, Fabry-Perot or Thorium-Argon lamp). The light from the two fibres is recorded onto a blue (380-520nm) and a red (520-775nm) detector. The instrument can operate in three different modes: High Resolution 1UT (HR), Ultra High-Resolution 1UT (UHR), and Medium Resolution 4UTs (MR). The main characteristics of these modes are summarised below (for more information about the characteristics of the instrument, see the Instrument Description).


HR (1UT) UHR (1UT)

MR (4UTs)

[not offered in P102]

Wavelength range 380–780 nm  380–780 nm  380–780 nm 
Resolving power (median) 140,000 190,000 70,000
Aperture on sky 1''.0 0''.5 4x1''.0
Global efficiency @ 550nm 9% 4% 9%
RV precision (requirement)
< 10 cm/s < 5 m/s < 5 m/s
Limiting V-band magnitude* ~17 ~16 ~20
Binning 1x1, 2x1 1x1 4x2, 8x4
Spectral sampling (average) 4.5 px 2.5 px 5.5 px (binned x2)
Spatial sampling per slice 9.0 (4.5) px 5.0 px 5.5 px (binned x4)
Number of slices 2 2 2

* based on approximate S/N per pixel <10 in one hour.

Project details

Principal Investigators

  • PI: Francesco Pepe (Observatoire de Genève)
  • Co-PI: Stefano Cristiani (INAF - Trieste)
  • Co-PI: Rafael Rebolo (IAC)
  • Co-PI: Nuno Santos (CAUP, U. do Porto)
Project responsibles
  • Project Manager: Denis Mégevand (Observatoire de Genève)
  • Project Scientist: Paolo Molaro (INAF - Trieste)
  • Observatoire de Genève and University of Bern (Switzerland)
  • INAF - Trieste and Brera (Italy)
  • Instituto de Astrofísica de Canarias (Spain)
  • Universidade do Porto, and Lisboa (Portugal)
  • European Southern Observatory (ESO)
Relevant references
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 Côte d'Azur, France), G. Tinetti (University College London, London, UK)
ESO responsibles


Project useful links