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ESPRESSO, a Rocky Exoplanets Hunter at the VLT
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Denis Mégevand , Francesco Pepe , Stefano Cristiani , Rafael Rebolo Lopez , Nuno C. Santos , José
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Miguel Herreros , Paolo Di Marcantonio , Alexandre Cabral , Filippo Zerbi , Christophe Lovis
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Université de Genève, Observatoire Astronomique, 51 ch. des Maillettes, 1290 Versoix Switzerland
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INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
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IAC, Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
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CAUP, Universidade do Porto, Centro de Astrofísica, Rua das Estrelas, 4150-762 Porto, Portugal
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Universidade de Lisboa, Laboratory of Optics, Lasers and Systems, F.C., Campus of Lumiar, Estrada do
Paço do Lumiar 22, 1649-038 , Lisboa, Portugal
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INAF, Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
ABSTRACT
ESPRESSO is a new high resolution, highly stable spectrograph for the VLT. It will inherit and enhance most
capabilities of HARPS and UVES, combining the efficiency of modern echelle spectrograph design with
extreme stability for unprecedented radial-velocity precision. The main scientific objectives will be the
detection 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, but many additional science cases will
benefit from its highly stable spectroscopic observations. The facility will be installed on ESO's VLT in order
to achieve a gain of two magnitudes with respect to its predecessor HARPS, and the instrumental radial
velocity precision will be improved to reach cm/s level. Furthermore, it will be located at the combined Coudé
focus of the VLT and may be linked to any of the four UT telescopes, enabling a great flexibility and an
efficient use of telescope time. Being able to combine incoherently the light of the four UTs in a lower
resolution mode, ESPRESSO will be the first instrument worldwide installed on the equivalent of a 16 meterclass telescope. This paper describes the instrument system and subsystems, enlightening the most
valuable differences between ESPRESSO and it's predecessors, the details of the project, entering now the
design phases, the ESPRESSO consortium, composed of Italian, Portuguese, Spanish and Swiss institutes,
and the relationship between the consortium and ESO.
1.
INTRODUCTION
A high-resolution, ultra stable, fiber-fed cross-dispersed echelle spectrograph, called ESPRESSO, is the
successor of a line of echelle spectrometers (CORAVEL, Elodie, Coralie, HARPS) designed to give high
accuracy radial-velocity (RV) measurements. It combines the best properties of two leading instruments,
enhancing the stability of HARPS and the resolution of UVES. It will be installed in the underground
Combined Coudé Laboratory (CCL) of the VLT, and will be linked to the four UT telescopes. ESPRESSO
stands for Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations. This
instrument is intended as an additional second-generation instrument for the VLT, as recommended by the
ESO STC during its 67th meeting, in October 2007 and the ESO council at its 11th meeting in December
2007.
2.
SCIENCE AND RETURN
2.1. Hunt for rocky exoplanets
The instrument will operate in different modes adapted to various science cases. The RV technique is one of
the most successful for the detection of low-mass exoplanets so far. As stated in [1], where the sciences
cases for ESPRESSO are detailed, HARPS is unveiling an exoplanet population that was not known before.
These discoveries were made possible thanks to the sub-m s-1 precision reached by this instrument. Given
the faint magnitude of the star and/or their tiny radial-velocity signal induced by the planet, most of the
observed objects would have remained out of reach of existing facilities that were limited to ~3m s -1.
One should expect a huge amount of still undiscovered planets, with smaller masses even in already
observed stellar samples. ESPRESSO is designed to explore this new domain and to enter into unknown
territories. This goal can only be obtained by combining high efficiency with high instrumental precision. In
the 1-UT mode, ESPRESSO may be linked to anyone of the four UT telescopes and thus benefit of possible
unused time to achieve those surveys with a great flexibility.
2.2. Variations of fundamental constants
Last decade has opened a debate on the real stability of the fundamental constants. Observation of very
faint QSO objects is crucial to determine with sufficient accuracy these variabilities. ESPRESSO, in its 4-UT
mode, will operate as a medium resolution spectrograph mounted on a 16 m equivalent telescope, giving
unprecedented results in this field. As a middle-term facility before the ELT class telescope, this mode will be
very appreciated by the astronomer in several fields.
2.3. Other science cases
Several other science cases have already been identified as possible recipients of the various possibilities
given by either the large aperture of the 4-UT mode, the ultra high stability of ESPRESSO or it’s very high
spectral resolution of R=225’000.
2.4. ESPRESSO as a CODEX precursor
ESPRESSO have been at first imagined as a prototype for the ultra accurate spectrograph for the European
Extremely Large Telescope (E-ELT), CODEX, conceptually designed under ESO leadership by almost the
same teams. Later, the first studies have shown that this prototype, installed at the VLT, could have thanks
its unique capabilities, a complete line of successful science results. Moreover, the size of the E-ELT is so
different from the VLT that it would have been impossible to simply clone ESPRESSO for that telescope.
Thus, the two instruments studies have been split in two unique instruments. ESPRESSO is now seen as a
precursor of CODEX in the sense that both instruments share some concepts and R&D developments, as
anamorphic optics, pupil slicing, pupil incoherent recombination, development of large echelle gratings, noncircular fibers for optical scrambling, advanced new wavelength calibrators, as Fabry-Perot cells and laser
combs, ultra-stable detector dewars.
2.5. Scientific Return
For the capital and human investment, the Consortium will be awarded Guarantee Time Observations. 80%
of the observing nights will be invested for the search and characterization of rocky planets in the habitable
zone of G, K, and M stars in the 1-UT mode. A 10% of the time will be dedicated to the determination of
possible variability of fundamental constants. Depending on the magnitude of the target, this program will be
carried out partially in the 1-UT, partially in the 4-UT mode. The remaining 10% will be reserved for
outstanding science cases and allocated as a function of topical questions arising at the moment of the GTO
Observations.
3.
INSTRUMENT DESIGN CONCEPTS
ESPRESSO is a high-resolution, fiber-fed cross-dispersed echelle spectrograph located in the CCL of the
VLT. Linked to the four 8.2 meters Unit Telescopes (UT), it may be operated in the 1-UT mode, where it is
illuminated by anyone of the four telescopes, either in standard or ultra-high resolution, or in the 4-UT mode,
where all the light from the four telescopes is recombined incoherently to reach fainter objects, with a
reduced resolution. The main characteristics of the instrument in the different modes are summarized in the
Table 1.
Table 1. Summary of the characteristics of ESPRESSO in its main modes
3.1. Light injection from the telescopes – Coudé-Train
The light is transmitted from each of the four UT telescope to this laboratory through four opto-mechanical
systems called Coudé-Trains (CT).
Figure 1. Schematic view of the VLT UTs respective positions and the ducts used for incoherent light
recombination at the CCL.
Each CT is composed partly by optical fibers and partly by optical components transmitting the light beam. In
the UT tubes, the light is transported from the Nasmyth focus to the Coudé room mainly via an optical fiber,
due to space constraints. Later, the light is transmitted in the air by optical elements through the tunnels.
These subsystems are to be developed in narrow collaboration with ESO due to the possible interferences
with the telescopes and the VLTI systems. Each CT is equipped with a primary guiding subsystem to ensure
proper injection in the CT fibers.
3.2. Front-End Unit
The Front-End (F/E) Unit receives and arranges adequately the beams arriving from the four UT through the
Coudé-Trains and the beams produced by the Calibration Unit – where all the calibration sources are located
and which produces the desired calibration light at each moment – and sends the light in the fibers linking it
to the spectrograph. To ensure the light to be properly centered in the output fibers, the F/E includes for each
incoming beam a secondary guiding camera and a tip-tilt mechanism. The F/E is also the place where the 1UT or 4-UT mode is selected.
Figure 2. Left: The Front-End unit with the 4-UT mode injection fiber. Right: The same F/E unit from rear side
with the 1-UT mode injection fiber
3.3. Optical design
The optical design is a clever mixing of several innovative designs, combining pupil slicing – used to
increase the resolution, and anamorphism – which enables to keep reasonable the dimensions of the main
disperser – the echelle grating, the collimating elements and the cameras.
Figure 3. The spectrograph optics, with the injection fiber arriving from the left, the echelle grating top right,
the blue camera bottom left and the red camera at its right.
In the figure 3, one can see on the left the fiber entering the spectrograph in the anamorphic pupil slicer
(APSU), the four bigger elements in the middle that are collimating mirrors, on top left the echelle grating and
in the lower part the two camera arms, blue on the left and red on the right, including the two crossdispersers. The figure 4 illustrates the various transformations of the image and the pupil in the optics of the
spectrograph.
Figure 4. Paraxial image and pupil sizes along the optical axis.
We are presently studying some possible simplifications and optimizations to the optical design. One
alternate concept could be the design shown in figure 5.
Figure 5. An alternate optical design
3.4. Temperature and Vacuum control
To achieve the required accuracy, ESPRESSO has to be highly stabilized both in temperature and pressure.
The temperature control is ensured by a triple thermal enclosure, which are seen as white exploded boxes in
figure 5. The more external one (1T) has a temperature stability of 1K, while the second stage (2T) ensures
0.1K stability. The third one (3T) is like an isolation box very close to the vacuum vessel and is stabilized to
0.01K.
Figure 6. Sketch of the vacuum vessel and the triple thermal enclosures.
Creating a vacuum that can be kept as low as possible by an effective pumping system ensures the stability
in pressure. The vessel is divided in three parts and is mounted on rails. That enables an easy opening for
maintenance purposes on different parts of the spectrograph. A two-stage pumping system with a secondary
turbo-molecular pump enables the stabilization of the vacuum inside the vessel to 5.10 -3 mbar and the
pumping of the detector unit at much higher vacuum of 10-6 mbar.
3.5. Control electronics and software
All these subsystems are controlled through appropriate control electronics (ICE) and control SW (CS) They
are developed conforming to the latest ESO standards. In particular, the CS is based as far as possible on
the ESO generic standard instrument packages (ICB – ICS Base and ic0) provided with the ESO VLT
common software kit.
The complete Data Flow System (DFS) including the CS and the science software is shown in the figure 7.
Figure 7. ESPRESSO Data Flow System.
3.6. Science software
The scientific requirements for the successful search for rocky planets make an Observation Preparation
Software (OPS) necessary to operate ESPRESSO in an easy and efficient way. The core of the Observation
Preparation Tool (OPT) will be a scheduler coupled with a local database and a user interface allowing the
support astronomer to choose the best-suited targets for the coming night or part of the night. The targets
will be chosen according to the observational and instrumental constraints and environmental conditions.
There will be no direct interface to any of the VLT software in Paranal.
Figure 8. The astronomer is the only link between the OPT and the ESO Paranal observation software.
The role of the Data Reduction Software (DRS) is to transform the raw data produced by the instrument into
reduced data of scientific quality. These represent the basic products sufficiently generic to be used as
inputs for further analysis of all main ESPRESSO science cases. The final products of the DRS process
have been found to be the extracted, background-subtracted, cosmic-corrected, flat-fielded and wavelengthcalibrated spectra.
The Data Analysis Software (DAS) is that part of the science software which is non-common to the various
scientific domains. Due to the very high stability and resolution foreseen for ESPRESSO, we would like to
supply the users with robust software pieces giving reliable results and not just ‘quick look’ numbers, for
most of the ESPRESSO science cases. When possible we will introduce automatic procedures and flexible
software pieces that can do the same operation for different kind of targets (e.g., quasars and stars).
4.
Project and management
4.1. Project network
The Consumer is the Astronomical Community of the member states of ESO. Astronomers of these
countries will be allowed to ask ESO for observing time with this instrument following exactly the same
procedure as for any other ESO instrument. ESO ensures the necessary technical and organizational
support. No exceptions or restrictions apply.
The Customer is ESO. On behalf of the Astronomical Community, ESO sub-contracts the realization of this
instrument to the Contractor. ESO specifies the Technical Requirements and establishes a Statement of
Work applicable to the Contractor, and verifies through Reviews held at various stages of the project that the
project is led in fulfillment with the defined technical and managerial frame. As customer, ESO supervises
the integration of the instrument and the software within the VLT environment.
The Science Team is distinguished from the Consortium. The Science Team is only linked to the instrument
by the GTO agreement with ESO. The Science Team composition ensures the scientific return for each of
the partners. Its composition will thus be directly linked to the investment each partner makes for the
construction project. An internal Memorandum of Understanding states the rules for the science team
constitution, competences and rights.
The contractor is a consortium described in next section.
4.2. Contractor consortium
The consortium involved in the Phase A study for ESPRESSO was composed of academic and research
institutes of four countries, Italy, Portugal, Spain and Switzerland, as listed in the Table 2. For the next
phases, we are presently studying an adaptation of the consortium structure in order to involve ESO as a
construction partner.
Table 2. The Phase A consortium institutes
Astronomical Observatory, University of Geneva
Switzerland
Physics Institute, University of Bern
Switzerland
Instituto de Astrofísica de Canarias, Tenerife
Spain
INAF – Osservatorio di Trieste
Italy
INAF – Osservatorio di Brera
Italy
Centro de Astrofisica da Universidade do Porto
Portugal
Facultade de Ciencias da Universidade de Lisboa – SIM
Portugal
Facultade de Ciencias da Universidade de Lisboa – LOLS-CAAUL Portugal
As for now, the Astronomical Observatory of the University of Geneva is the head institute of the consortium,
where the principal investigator (PI) and the project manager (PM) are located. The system engineer (SE) is
located in the Instituto de Astrofísica de Canarias and the software system engineer (SSE) in the Trieste
Observatory.
4.3. Work breakdown structure
Figure 9. The product tree’s inherited work breakdown structure (WBS) is composed of various
workpackages (WP).
Each WP is attributed to a WP manager (WPM), and the tasks linked to this WP are executed in the WPM
institute. This structure has the advantage to minimize the interfaces between institutes, as they are only
external to the WP. The various WPs identified during phase A are listed in the Table 3, with the responsible
WPMs and institutes:
Table 3. Workpackages with the responsible individuals and institutes.
VLT Interface
AVILA Gerardo
ESO
Coudé Train Optics
CABRAL Alexandre
LOLS-CAAUL
Coudé Train Mechanics
MOITINHO André
SIM
Front-End Unit, Calibration Unit
ZERBI Filippo
INAF BRERA
Fiber Link
RASILLA José Luis
IAC
Vacuum and Thermal System
FLEURY Michel
OBS GENEVA
Spectrometer Optics and Exposure Meter SPANÓ Paolo
INAF BRERA
Spectrometer Opto-mechanics
TENEGI Fabio
IAC
Scientific Detectors
IWERT Olaf
ESO
Detector Unit
LIZON Jean-Louis
ESO
Control Electronics
COMARI Maurizio
INAF TRIESTE
Control Software
DI MARCANTONIO Paolo INAF TRIESTE
Observation Software
SANTIN Paolo
INAF TRIESTE
Observation Preparation Software
SOSNOWSKA Danuta
OBS GENEVA
Data Reduction Software
LOVIS Christophe
OBS GENEVA
Data Analysis Software
D'ODORICO Valentina
INAF TRIESTE
4.4. Planning
The start of the project was preceded by a conceptual study called Phase A during which the Consortium
translated the scientific requirements into general technical requirements. This phase ended in March 2010
by a review during which the baseline design of the instrument and general SW functions were presented to
the ESO management.
We completed a quite extensive conceptual study Phase A in March 2010. The main consequence is that
the upcoming preliminary design phase following this Phase A is foreseen as rather short; this phase is
called Phase A’. The milestones of the project are presented in Table 4 and the Figure 9 the general Gantt
chart of the project, including the proposal preparation and concept study phases.
Table 4. Project milestones
Milestone
From T0
Planned Date
Site
Study Kick-off
TS
26-27/01/2009
IAC, Tenerife
Phase A review
TS + 14 months 18-19/03/2010
Project Kick-off
TP
4th quarter 2010
Preliminary Design Review
TP + 6 months
2nd quarter 2011
Optics Final Design Review
TP + 15 months 1st quarter 2012
Final Design Review
TP + 24 months 4th quarter 2012
CT Instrument Readiness
TP + 38 months 4th quarter 2013
Instrument Readiness
TP + 42 months 1st quarter 2014
ESO Garching
ESO Garching
CT Preliminary Acceptance Europe TP + 44 months 2nd quarter 2014
Preliminary Acceptance Europe
TP + 50 months 4th quarter 2014
Preliminary Acceptance Chile
TP + 57 months 3rd quarter 2015
ESO Paranal
Final Acceptance Chile
TP + 90 months 2nd quarter 2018
ESO Paranal
The Phase A’ will be the consolidation of the options taken during Phase A and accepted at the review, the
reworking and adaptation of the documents presented at Phase A to take into account the conclusions of
Phase A, to transform the concepts in technical specifications and requirements on the SW. This Phase A’
will conclude by a Preliminary Design review, where also all R&D aspects should be rather advanced, either
closed or acceptable alternatives identified.
The Phase B will be the final design phase. At that point, the design should be proven compliant with the
requirements.
The manufacturing, assembly and tests of the subsystems will take place in the phase C.
The Phase D is the integration and verification of the whole system in Europe, before the instrument is
transported, installed and commissioned at Paranal during Phase E.
4.5. Managerial structure
The PI is the single point of contact between the Customer, ESO and the Contractor consortium for all
contractual matter.
Each partner country is represented in the managerial structure by a PI or a co-PI, which is the single point
of reference for the institutes located in his country and responsible for the financial contribution of his
country and the related institutes.
The PI and co-PIs form the executive board, responsible for all managerial matter. It has therefore the
authority to take strategic and organizational decisions about the project.
The PM is responsible for the successful execution of the project. He takes the relevant decisions within the
scientific, managerial, and technical frame defined by the Scientific Program, the Statement of Work, and the
Technical Requirements Specification.
The System Engineer leads a System Team (ST) that has the responsibility to ensure the proper
development of the instrument at the technical level. The System Engineer has to ensure that the technical
requirements specifications at system level are met. He supervises the engineering tasks and verifies the
consistency between the design and the technical specifications.
Figure 10. Managerial structure.
The Project Scientist (PS) leads a Science Advisory Team (SAT) that helps the PIs in the definition of the
scientific program and monitors the evolution of the scientific program and translates this evolution in terms
of technical system-level requirements. In particular, one member of the team will be in charge of the
instrument-oriented tasks and will have the status of Instrument Scientist.
The Management Board, which consists of the PI, the CO-PIs, the PM, the SAT, the ST, the ESO
Representative, and their respective deputies take strategic decisions. In order to ensure that all members
remain continuously and
The ESO Representative represents ESO within the Executive Board. He takes part in all discussions and
decision related to the project excluding matter related to the scientific return to the Consortium. The goal is
to ensure the highest degree of communication between the Consortium and ESO during all phases of the
project and avoid or identify possible conflicts as soon as possible. Moreover, the ESO Representative will
assume the same “individual responsibilities” as the PI and co-PIs for all work-packages that by contract are
under ESO responsibility.
4.6. Role of ESO
Apart form being the customer of the instrument, ESO will play a role as a full partner during the
development of the instrument. It will thus be integrated as a consortium member. Several areas of activities
of the project (work packages) will be under the direct responsibility of ESO, since a different solution would
not have been accepted by ESO and would lead to much more complex solutions. These work packages will
be fully integrated into the global project, will have a responsible person at ESO and will be defined by tasks,
schedule and deliverables. These WP are more specifically the management of the interface with Paranal,
the development of the detector unit, the scientific detectors, the lasercomb, the procurement of the echelle
grating and of the two cameras, and the interface management with the VLT at Paranal.
5.
Conclusions
ESPRESSO is
• A super-HARPS on a 10 m-class telescope,
• A spectral coverage from 380 to 800 nm in one shot,
• A wavelength calibration far more accurate than any other facility,
• An instrument producing cleanest, best-quality spectra, both at high and low SNR,
• A spectrograph on a 16 m telescope, the largest visible photon-collector until ELTs will be available,
• An ultra-high resolution mode (R~225,000), far beyond other existing facilities on a 10 m-class telescope,
• Fitting in standard VLT operations as far as possible.