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ESPRESSO: The next European exoplanet hunter
F. Pepe, P. Molaro, S. Cristiani, R. Rebolo, N. C. Santos, H. Dekker, D. Mégevand, F. M. Zerbi, A. Cabral, P. Di Marcantonio, M. Abreu, M. Affolter, M. Aliverti, C. Allende Prieto, M. Amate, G. Avila, V. Baldini, P. Bristow, C. Broeg, R. Cirami, J. Coelho, P. Conconi, I. Coretti, G. Cupani, V. D'Odorico, V. De Caprio, B. Delabre, R. Dorn, P. Figueira, A. Fragoso, S. Galeotta, L. Genolet, R. Gomes, J. I. González Hernández, I. Hughes, O. Iwert, F. Kerber, M. Landoni, J. -L. Lizon, C. Lovis, C. Maire, M. Mannetta, C. Martins, M. Monteiro, A. Oliveira, E. Poretti, J. L. Rasilla, M. Riva, S. Santana Tschudi, P. Santos, D. Sosnowska, S. Sousa, P. Spanó, F. Tenegi, G. Toso, E. Vanzella, M. Viel, M. R. Zapatero Osorio
TL;DR
ESPRESSO addresses the need for highly efficient, extremely precise spectroscopy to search for rocky exoplanets and study possible variations in fundamental physical constants. It links the VLT’s Unit Telescopes to a stable high-resolution spectrograph, achieving 10 cm s−1 radial-velocity precision and expanded observational reach, while some ultra-small signals remain beyond its sensitivity.
Problem
The project targets precise searches for rocky planets around nearby quiet G to M dwarfs and investigations of fundamental-constant variability, alongside broader high-resolution studies requiring greater sensitivity.
Method
ESPRESSO combines a high-resolution ultra-stable spectrograph with single- or multi-telescope VLT operation, laser-frequency-comb calibration, integrated data flow, and fibre links supporting multiple observing modes.
Results
10 cm s−1 radial-velocity precision enables detection of Earth-mass rocky planets in habitable zones around solar-type stars, while 4UT operation reaches approximately 1.5 magnitudes fainter than UVES.
Takeaways & Limitations
ESPRESSO broadens precision and sensitivity for exoplanet searches and other astrophysical programmes, with full-quality scientific data delivered less than a minute after observations.
Takeaways & Limitations
ESPRESSO is probably not sufficiently sensitive in 4UT mode to measure cosmological redshift drift signals at the level of a few cm s−1 yr−1.
Abstract
from arXiv · showhide
The acronym ESPRESSO stems for Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations; this instrument will be the next VLT high resolution spectrograph. The spectrograph will be installed at the Combined-Coudé Laboratory of the VLT and linked to the four 8.2 m Unit Telescopes (UT) through four optical Coudé trains. ESPRESSO will combine efficiency and extreme spectroscopic precision. ESPRESSO is foreseen to achieve a gain of two magnitudes with respect to its predecessor HARPS, and to improve the instrumental radial-velocity precision to reach the 10 cm/s level. It can be operated either with a single UT or with up to four UTs, enabling an additional gain in the latter mode. The incoherent combination of four telescopes and the extreme precision requirements called for many innovative design solutions while ensuring the technical heritage of the successful HARPS experience. ESPRESSO will allow to explore new frontiers in most domains of astrophysics that require precision and sensitivity. The main scientific drivers are the search and characterization of rocky exoplanets in the habitable zone of quiet, nearby G to M-dwarfs and the analysis of the variability of fundamental physical constants. The project passed the final design review in May 2013 and entered the manufacturing phase. ESPRESSO will be installed at the Paranal Observatory in 2016 and its operation is planned to start by the end of the same year.
1 Introduction
High-resolution spectroscopy increasingly requires both sensitivity and long-term precision, motivating ESPRESSO as an ultra-stable VLT spectrograph developed from the successful HARPS experience.
- High-resolution spectroscopy supports physical studies of stars, galaxies, and interstellar and intergalactic media, while requiring observations of fainter objects and greater precision.
- HARPS pioneered precise radial-velocity measurements, while the search for habitable-zone terrestrial planets motivated a similar instrument on the VLT.
- Ten years of HARPS radial-velocity measurements of τ Ceti show an overall dispersion of 1 m s−1.
- ESPRESSO was proposed as a high-resolution, ultra-stable spectrograph for the VLT combined-Coudé focus and was selected for construction by the ESPRESSO Consortium in collaboration with ESO.
2 What science?
ESPRESSO is designed to extend high-precision spectroscopy across rocky-planet searches, fundamental-constant tests, stellar and galactic chemistry, and other astrophysical investigations. Its 10 cm s−1 radial-velocity precision and high efficiency support studies ranging from Earth-mass habitable-zone planets to faint metal-poor stars and quasar absorption systems.
- ESPRESSO’s main scientific drivers are high-precision radial-velocity searches for rocky planets, measurements of varying physical constants, and chemical analysis of nearby stars.
- Searching for rocky planets in the habitable zone: 10 cm s−1 radial-velocity precision could detect Earth-mass rocky planets in the habitable zones of solar-type stars.The expected precision is about ten times better than HARPS; lower-mass stars produce larger radial-velocity signals.
- Searching for rocky planets in the habitable zone: ESPRESSO will support characterization of very light planets, transit follow-up, transit spectroscopy, reflected-light studies, and stellar-oscillation measurements.Its observations can provide candidates for follow-up with transit, astrometric, and Rossiter-McLaughlin techniques.
- A scientific Pandora box: ESPRESSO will provide chemical information for nearby galaxies and exquisite spectra of extremely metal-poor stars, while expanding access to faint quasar pairs.In 4UT mode, it is expected to reach about 1.5 magnitudes fainter than UVES, nearly increasing observable close quasar pairs twentyfold.
- A scientific Pandora box: The instrument combines unprecedented radial-velocity and spectroscopic precision with ESO’s largest photon-collecting area and resolving power R ∼200 000.
3 A new-generation instrument for the VLT
ESPRESSO is a fibre-fed, high-resolution spectrograph in the VLT Combined Coudé Laboratory, receiving light from one or up to four Unit Telescopes through Coudé trains.
- ESPRESSO is a fibre-fed, cross-dispersed, high-resolution échelle spectrograph located at the VLT Combined Coudé Laboratory.Its front-end can combine light from up to four Unit Telescopes at the incoherent focus.
- The instrument receives telescope light through Coudé-train optics and uses separate target and sky fibres to form the spectrograph slit.
- The Front-End and spectrograph layouts are replicated or arranged to support the arrival and processing of beams from the VLT telescopes.
- ESPRESSO receives light from any of the four Unit Telescopes through a newly implemented Coudé focus and modified Paranal infrastructure.
3.1 The Coud´e train
The Coudé train transports each telescope beam from its Nasmyth focus through the underground Coudé rooms to the Combined Coudé Laboratory, where the beams are combined and conditioned.
- Four optical trains relay a 17 arcsec field from the Unit Telescopes toward the Combined Coudé Laboratory over 48–69 m distances.
- Each train uses six powered prisms and two large lenses to convey light from the Nasmyth focus through the Coudé room toward the laboratory.
- At the Combined Coudé Laboratory, the four beams meet for observing-mode selection and beam conditioning in the Front-End fore-optics.
3.2 The Front-End
The Front-End corrects and stabilizes incoming beams before fibre injection, while the Fibre-Link forms the pseudo-slit and supports distinct one- and four-telescope configurations.
- The Front-End applies atmospheric-dispersion correction, pupil and field stabilization, focusing, and calibration-light injection before fibre feeding.Pupil and field stabilization use independent technical-camera and tip-tilt control loops.
- The Fibre-Link relays light to the vacuum vessel and forms the spectrograph pseudo-slit from object and sky/reference fibres.
- In 4-UT mode, four object fibres and four sky/reference fibres are combined into single square 280 µm fibres for the spectrograph.
- The Front-End's optical path includes the pupil and fibre-image locations used to characterize beam propagation through the spectrograph.
- A passive toggling mechanism selects among the available observational modes.
3.3 Observing modes
ESPRESSO uses separate target and reference fibres to support calibrated precision measurements in bright-source mode and sky-background handling in faint-source mode.
- The target fibre receives either astronomical-object light or calibration light, while the reference fibre receives sky or calibration light depending on observing mode.
- Non-circular fibre shapes improve light scrambling and illumination stability for the spectrograph.
- In bright-source mode, simultaneous reference tracks instrumental drifts down to the cm s−1 level, with measurements limited by photon noise.
- In faint-source mode, detector noise and sky background may become significant, so the second fibre supports their treatment.
3.4 Performances
ESPRESSO’s performance is characterized across single- and multi-telescope observing modes using signal-to-noise projections and radial-velocity precision estimates.
- SNR = 10 per extracted pixel is obtained in 20 minutes on a V = 16.3 star in singleHR mode at R ≈134 000.
- 10 cm s−1 RV precision is estimated for a non-rotating K5 star at SNR = 540, corresponding to V = 8.6 in singleHR mode.
- For an F8 star, the same 10 cm s−1 precision is estimated at V = 8.
- At R ≈60 000 in multiMR mode, a SNR of ≈10 is achieved on a V = 19.4 star with a 20 minute exposure.
3.5 Design
ESPRESSO’s optical design combines pupil shaping, spectral splitting, optimized dispersion, and fast cameras to reduce instrument size while preserving efficiency across observing modes.
- The performance figures show achievable SNR as a function of stellar visible magnitude for singleHR and multiMR observing modes.
- The APSU reduces the required echelle-grating size from 240×40 cm to 120×20 cm, with smaller collimators and cross-dispersers.
- Heavy binning is planned for faint-object observations, especially in 4-UT mode, to avoid increased detector noise from the elongated doubled spectrum.
- The APSU compresses the pupil in cross-dispersion and splits it into two beams superimposed on the echelle grating.
- A dichroic separates the broad spectral range into blue and red arms, allowing each arm to be optimized for image quality and optical efficiency.
- VPHGs separate spectral orders while compressing order height to maximize interorder spacing and SNR per pixel.
3.6 The opto-mechanics
ESPRESSO uses a fixed, highly stable opto-mechanical configuration to target radial-velocity precision of approximately 10 cm s−1.
- 10 cm s−1 RV precision is the design target, approximately one order of magnitude better than HARPS.
- The optics are mounted on a three-dimensional optical bench designed to maintain the thermo-mechanical tolerances required for high-precision RV measurements.
3.7 Large-area CCDs
ESPRESSO employs large monolithic CCDs and specialized packaging to cover its broad optical field while supporting stability and precise detector operation.
- Sixteen high-speed output ports provide fast readout of the large monolithic CCD.
- The pseudo-slit layout simultaneously represents standard-resolution 1-UT, ultra-high-resolution 1-UT, and mid-resolution 4-UT fiber sets.
- More than 80% of encircled energy falls within the 10 µm CCD pixel size at nine wavelengths for all orders in the blue camera.
- The CCD package uses silicon carbide, while the aimed 10 cm s−1 rms precision corresponds to measuring 2 nm line-position changes in the CCD plane.
3.8 A laser frequency comb
ESPRESSO uses a laser frequency comb as its baseline wavelength-calibration and simultaneous-reference source because existing spectral sources lack the required breadth, richness, stability, and uniformity. A stabilized Fabry–Pérot is being developed as a backup.
- Simultaneous reference: ESPRESSO will record a spectral reference simultaneously on the scientific detector to track residual instrumental drifts.This technique follows the approach adopted in HARPS.
- Calibration source: A laser frequency comb is ESPRESSO’s baseline source for wavelength calibration and simultaneous reference.It provides a link to the frequency standard.
- Calibration source: Existing thorium-argon lamps and iodine cells do not provide spectra sufficiently wide, rich, stable, and uniform for ESPRESSO’s requirements.The required wavelength-calibration repeatability is approximately Δλ/λ ≈ 10^-10.
- Backup solution: A stabilized Fabry–Pérot is under development as a backup solution to minimize calibration risks.The laser frequency comb procurement covers the full 380–760 nm range.
4 ESPRESSO’s data flow
ESPRESSO’s data flow integrates observation preparation, instrument control, data reduction, and analysis to deliver science-ready results rapidly. The system supports flexible use of the four Unit Telescopes and targets demanding radial-velocity programmes.
- Integrated data flow: ESPRESSO is designed as a science-generating machine rather than a standalone instrument, with an integrated software-cycle view from preparation through analysis.The design aims to improve compatibility, operations, maintenance, and scientific output within the ESO Paranal Data Flow environment.
- Observation preparation: The Observation Preparation Software helps observers select suitable targets and adjust parameters for planet-search observations.It communicates directly with the Visitor Observing Tool and supports scheduling at the telescope.
- Data analysis: The Data Analysis Software provides automated recipes for stellar and quasar spectra, including atmospheric-parameter estimation and absorption-system identification.It uses existing ESO tools based on CPL and is compatible with Reflex.
- Data reduction: The Data Reduction Software is intended to deliver high-quality, science-ready data shortly after observations, including radial-velocity computation better than 10 cm/s.It must handle the instrument signature, simultaneous reference, complex FITS data, and multi-UT operation.
- End-to-end operation: Any combination of the four Unit Telescopes can feed ESPRESSO in singleHR mode, improving scheduling flexibility and VLT-time use.This flexibility benefits radial-velocity planet searches and time-critical programmes such as transiting-planet studies.
- End-to-end operation: ESPRESSO is intended to deliver full-quality scientific data less than a minute after an observation ends.The stated goal depends on full integration of the data-flow system.