espresso projecting a rock y exoplanet hunter for the vlt · 2011-03-03 · 1.1 hunt for rocky...

ESPRESSO – Projecting a rocky exoplanet hunter for the VLT

Denis Mégevand*a, José-Miguel Herrerosb, Filippo Zerbic, Alexandre Cabrald, Paolo Di Marcantonioe, Christophe Lovisa, Francesco Pepea, Stefano Cristianie, Rafael Rebolob, Nuno C.

Santosf

aObservatoire de l’Université de Genève, Ch. des Maillettes 51, CH-1290 Versoix, Switzerland

bInstituto de Astrofísica de Canarias, C/. Vía Láctea, E-38200 La Laguna, Tenerife, Spain cINAF – Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy

dLOLS-CAAUL, Faculty of Sciences, University of Lisbon, Estrada do Paço do Lumiar 22, 1649-038 Lisboa, Portugal

eINAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34143 Trieste, Italy fCentro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal

ABSTRACT

ESPRESSO is a high-resolution, highly stable spectrograph for the VLT. It will inherit and enhance most capabilities from HARPS and UVES, combining both stability and efficiency. The main science driver will be the detection and characterization of Earth-like planets, but many additional science cases will benefit from its highly stable spectroscopic observations. The facility will be installed at the combined Coudé focus of the VLT and may be linked with any of the four UT telescopes, enabling thus a great flexibility for the efficient use of telescope time. This particularity makes the interface with the VLT more complex than for an instrument fed by a single telescope. It impacts on the complexity of the relationship between the consortium providing the instrument and ESO, the customer. The targeted high RV accuracy requires very high performances in stability and resolution, which in turn require adequate technical solutions at several levels. 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.

Keywords: Spectrograph, high resolution, high stability, VLT, radial velocity, exoplanets

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.

1.1 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.

*[email protected]; phone 41-22-379-2353; fax 41-22-379-2205; www.unige.ch/sciences/astro

Ground-based and Airborne Instrumentation for Astronomy III, edited by Ian S. McLean, Suzanne K. Ramsay, Hideki Takami, Proc. of SPIE Vol. 7735, 77354Y · © 2010

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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.

1.2 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.

1.3 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. F. Pepe et al. give a list of these science cases in [1].

1.4 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, non-circular fibers for optical scrambling, advanced new wavelength calibrators, as Fabry-Perot cells and laser combs, ultra-stable detector dewars.

2. 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



In the next sections, we present an overview of the main subsystems characterizing the instrument.

2.1 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. The Coudé-Train are described in detail by A Cabral et al. in [2].

2.2 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 1-UT 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



2.3 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 echelle grating top right, the blue camera bottom left and the red camera at 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 cross-dispersers. The figure 4 illustrates the various transformations of the image and the pupil in the optics of the spectrograph. P. Spano et al. detail in [3] the optical design of ESPRESSO.

Figure 4. Paraxial image and pupil sizes along the optical axis.

2.4 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 5. 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.

2.5 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 6.

Figure 6. ESPRESSO Data Flow System.



2.6 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 7. 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 wavelength-calibrated 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).

3. PROJECT AND MANAGEMENT 3.1 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.



3.2 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.

3.3 Work breakdown structure

Figure 8. 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

3.4 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 ESO Garching

Project Kick-off TP 3rd quarter 2010

Preliminary Design Review TP + 6 months 4th quarter 2010

Optics Final Design Review TP + 15 months 3rd quarter 2011

Final Design Review TP + 24 months 2nd quarter 2012 ESO Garching

CT Instrument Readiness TP + 38 months 3rd quarter 2013

Instrument Readiness TP + 42 months 4th quarter 2013

CT Preliminary Acceptance Europe TP + 44 months 1st quarter 2014

Preliminary Acceptance Europe TP + 50 months 3rd quarter 2014

Preliminary Acceptance Chile TP + 57 months 2nd quarter 2015 ESO Paranal

Final Acceptance Chile TP + 90 months 4th quarter 2017 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.



Figure 9. Folded general GANTT chart of the ESPRESSO project.

3.5 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 9. 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, the ESO management, technical and scientific contacts and their respective deputies take strategic decisions. In order to ensure that all members remain continuously and

The ESO Representative represents ESO within the Management Board. He takes part in all discussions and decision related to the project excluding matter related to the contractual relation between ESO and 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.

3.6 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 and 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 and of the scientific detectors, and the integration and tests of the Coudé-Trains at Paranal.

ESO will also play a capital role in the funding of the instrument. We expect ESO to contribute to the direct capital expenses in the following areas: laser comb development, contribution to the optics, the detector units and the detectors themselves, Paranal preparation costs.

REFERENCES

[1] Pepe, F. A. et al, “ESPRESSO: the Echelle spectrograph for rocky exoplanet and stable spectroscopic observations”, Proc SPIE 7735, Paper 14 (2010).

[2] Cabral, A. P. et al., “ESPRESSO: Design and analysis of Coudé-Train concepts for stable and efficient optical feeding”, Proc. SPIE 7739, Paper 178 (2010).

[3] Spanò, P. et al., “Optical design of the ESPRESSO spectrograph at the VLT”, Proc. SPIE 7735, Paper 20 (2010).



espresso projecting a rock y exoplanet hunter for the vlt · 2011-03-03 · 1.1 hunt for rocky...

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