ESPRESSO: A High Resolution Spectrograph for the Combined Coudé Focus of the VLT

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  In the frame of the call for proposal for the E-ELT instrumentation studies, the CODEX team carried out the feasibility study for a spectrograph for extremely stable Doppler measurements. The CODEX project and its scientific goals are described in
  ESPRESSO: A High Resolution Spectrographfor the Combined Coudé Focus of the VLT Luca Pasquini, A. Manescau, G. Avila, B. Delabre, H. Dekker, J. Liske,S. D’Odorico, F. Pepe, M. Dessauges, C. Lovis, D. Megevand, D. Queloz,S. Udry, S. Cristiani, P. Bonifacio, P. Dimarcantonio, V. D’Odorico, P. Molaro,E. Vanzella, M. Viel, M. Haehnelt, B. Carswell, M. Murphy, R. Garcia-Lopez,J.M. Herreros, J. Perez, M.R. Zapatero, R. Rebolo, G. Israelian, E. Martin,F. Zerbi, P. Spanò, S. Levshakov, N. Santos and S. Zucker 1 The Drivers In the frame of the call for proposal for the E-ELT instrumentation studies, theCODEX team carried out the feasibility study for a spectrograph for extremelystable Doppler measurements. The CODEX project and its scientific goals are de-scribed in [6]. During the development of this study, the CODEX team recognizedthat a CODEX-like instrument would be of high scientific interest also on the VLT.The ESPRESSO concept was born. The contribution by J. Liske in this volumehighlights the direct links between the two instruments (see also [4]).The ESPRESSO concept evolves from previous positives experiences at ESO,combining the stability of HARPS [3] with the efficiency of UVES and FEROS[1, 2]. In summary, it is a high-efficiency, high-resolution, fiber-fed spectrograph of  highmechanicalandthermalstabilityusing,ifnecessary,thesimultaneousreferencetechnique.ThefirstpurposeofESPRESSOistobeacompetitive,innovativehigh-resolutionspectrograph to fully exploit the VLT potentiality and to allow new science.ESPRESSO has indeed many very interesting applications, and several have beenaddressed in this conference by different speakers. The quest for enhanced radialvelocity capabilities at the VLT for exo-planet search has been emphasized by Ren-zini, Bono, Queloz and Udry. P. Molaro discussed the relevance of investigating thevariability of physical constants, and V. D’ Odorico the results which can be ob-tained by studying the chemistry of the Intergalactic Medium. Finally, the detailedchemical analysis of stars will greatly benefit from ESPRESSO, as highlighted, forinstance in the presentations by Bonifacio. Additional interesting topics are widelydiscussed in the proceedings of the “Precision Spectroscopy in Astrophysics” con-ference [7]. L. Pasquini (  )ESO, Garching, Germanye-mail: lpasquin@eso.orgA. Moorwood (ed.),  Science with the VLT in the ELT Era, Astrophysics and Space Science Proceedings ,doi: 10.1007/978-1-4020-9190-2_68, © Springer Science + Business Media B.V. 2009395  396 L. Pasquini et al. The second purpose of ESPRESSO is to gather fundamental experience forCODEX.We finally find extremely exciting the possibility of using a 16 m equivalenttelescope, in advance the E-ELT will be fully available to the community. 2 From Requirements to Design The first challenge is to  obtain the highest stability, while preserving an excellentefficiency . High spectrograph optomechanical stability is obtained through a con-trolled environment in vacuum and avoiding movable components. One critical itemis the light input system, which must scramble the signal to ensure that the variabil-ity at the fiber input does not degrade the stability of the spectrograph, still keepingan excellent transmission. The requirement that ESPRESSO is kept in vacuum andthermally controlled implies containing the instrument volume and the optics size. Obtaining the results from an integrated, system perspective:  in order toobtain the demanding ESPRESSO performances, the whole chain must work, fromobject acquisition, to the data reduction and analysis. While a definitive answer willeventually come only from the use of the instrument, two main system tools havebeen adopted: first tool is the capitalization of the HARPS experience and its exten-sion to the ESPRESSO requirements.Second tool is the extensiveuse of simulationsgenerated to quantify the calibration requirements, the main requirements, the sub-system requirements.A number of critical items have been identified, and they are addressed withdedicated R&D efforts:1. Efficiency: Improvements in efficiency with respect to HARPS include a shorterfiber length, a more efficient scrambling system and a two arm spectrographdesign, with the use of two VPHs as crossdispersers.2. Scrambling: This aspect differs between the 1-UT case and the 4-UTs case. Inthe 1-UT case the problem is analogue to the one of HARPS, but with 50 timesmore stringent requirements. In the 4-UT case there are 4 independent pupils; thescrambling shall therefore happen after combining the light from the sub-pupils,and to this scope 3 different systems are tested.3. CCD temperature control: HARPS shows a correlation between detector temper-atureandradialvelocityvariations.Acopyof theHARPScryostatisstudied,anddesign changes are made to improve this aspect. The construction of a prototypeis planned.4. Calibration System: A novel calibration system based on a laser frequency combhas been proposed and its feasibility is studied through a contract with MPQ.The final prototype is expected in 3 years from now (cf. Manescau et al., theseproceedings).5. Slanted VPHG: The slanted fringes VPHG is a potential innovative feature since,to the best of our knowledge, such a VPHG has never used in combination with aspectrograph, as crossdisperser and beam compressor. The use of slanted VPHs  Title Suppressed Due to Excessive Length 397 is not mandatory for ESPRESSO, but seems unavoidable for CODEX. We havetherefore opted to proceed to the prototyping. 3 The Design Following HARPS, ESPRESSO is designed with a dual fiber system, where thesecondfibercanbeusedeithertorecordtheskyortomonitorthespectrographshiftsby recording a simultaneous calibration source. It is actually one of the ESPRESSOaims to gather sufficient information for deciding if a simultaneous calibration fiberis required in CODEX, or if an operations scheme with interloped calibrations isacceptable for that instrument. ESPRESSO will sit in the Combined Coudé room,fed by a Coudé train which brings the light from the B Nasmyth platform to theroom, where it is collected by the instrument acquisition and guide system into afiber,whichfeeds the spectrograph.The spectrograph itself is containedin a vacuumtank enclosed in a thermally controlled room. No movable nor motorized functionsare present inside the tank.  3.1 The Coudé Train (see Avila et al.) Even if in the VLT provision for the Coudé optics is made, the design and hencethe components of the train require to be developed. The ducts distance from eachtelescope to the combined focus is different for each UT; even if the concept for theCoudé train is the same for all UTs, each one will be different. The Coudé train hasa FoV of 5 arcsec radius and it will be coated for an excellent response in the 300–750 nm wavelength range. Given the large distance traveled by the light in the duct,some induced seeing is expected at the fiber entrance, which will be compensatedby a stabilization system in the fiber head . This component is critical, because theHARPS experience has shown as, even with a good scrambler, a movement of thesource of 0.5 arcsec may induce a shift of up to a few m / sec.  3.2 Calibration Unit (see Manescau et al.) The relevance of a novel, precise, predictable stable calibration system cannot beover-stressed and the characteristics of an ideal calibration source are described,for instance in [5]. The wavelength calibration unit for the spectrograph is basedon Laser Frequency Comb system. Provision for the use of Th-Ar lamps and otherlamps for flat field will be made.  398 L. Pasquini et al. Fig. 1  The optical layout of ESPRESSO. The light, after injection form the fibers, passes througha pupil anamorphoser and the pupil is split. The two half-pupils are projected onto the echelle.The dichroic separates the  blue  and  red   arms, which are crossdispersed by VPH gratings. A colourversion of this figure is available at  3.3 Spectrograph Optics (see Spanò et al.) The basic concept for the spectrograph is a cross dispersed echelle with two arms.The echelle grating size is 1700 × 200 mm, consisting of a 4 × 1 mosaic of 408 × 200 mm grating segments, or two UVES gratings. To limit the size of the echelle,pupil slicing is applied. The grating operates in near Littrow configuration.The optical design makes use of anamorphism and pupil slicing. In this way acompact design is obtained, and a 20 cm optical beam and a 20 × 160 cm echelleprovides the resolving power of a un-sliced 40 cm beam spectrograph. The spectro-graph optical design is given in Fig. 1.  3.4 Mechanics & Vacuum Vessel (see Zapatero and Osorio et al.) The spectrograph mechanics shall maintain the correct alignment and the configura-tion has been chosen to allow highest stability providing easy access for installationand maintenance. The whole spectrograph, including the detector head, is mountedon an optical bench and installed within a evacuated vacuum vessel with its temper-ature precisely controlled (few mK). The vacuum tank will be hosted in a thermallycontrolled room in the Coudé lab.  Title Suppressed Due to Excessive Length 399 Table 1  Characteristics of the ESPRESSO designCharacteristic Standard 1 UT Faint object 4 UT High efficiency 4 UTWavelength range 350–780 nm 350–780 nm 350–780 nmResolving power 160000 40000 80000Sampling (average) 4 pixels 16 pixels 8 pixelsSpatial pixels 24 24 48Simultaneous calibration YES NO YESSky subtraction YES (either SimCal) YES YES (either SimCal)  3.5 Instrument Control and Software (see Megevand et al.) The instrument control hardware consist in a number of LCUs and the different con-trollers to control the instrument functions. The Data Reduction Software will trans-form raw frames into clean, extracted, flat fielded, wavelength calibrated spectra.The main difference between the ESPRESSO DRS and the standard ESO pipelinesis that it shall deliver the best science quality data on-line. Data Analysis SW is thatpart of the science SW which is non-common to the various scientific domains andrequires specific tools and focus. The Data-Analysis SW is preferably automatic.For all tools the input is an extracted, wavelength-calibrated spectrum delivered bytheDRS,andits outputare scientificobservables.We distinguishfollowingdomainsfor which different data-analysis tools/functions are required: •  Analysis of non-stellar spectra, low SNR science. •  Correlation, RVs, bi-sector analysis. •  General (stellar) spectroscopy. •  Package of other RV extraction methods. 4 Performances ESPRESSO is proposed to have three operating modes, one with 1-UT and two with4-UTs. The summary of the characteristics is given in Table 1. References 1. H. Dekker, S. D’Odorico et al., SPIE  4008 , 534 (2000)2. A. Kaufer, L. Pasquini, SPIE  3355 , 844 (1998)3. M. Mayor, F. Pepe et al., Messenger  114 , 20 (2003)4. J. Liske, A. Grazian, E. Vanzella et al., 2008MNRAS. tmp..460L, in press5. M. Murphy, T. Udem et al., Mon. Not. R. Astron. Soc.  379 , 1407 (2007)6. L. Pasquini, S. Cristiani et al., Messenger  122 , 10 (2005)7. N. Santos, L. Pasquini, A. Correia, M. Romaniello,  Precision Spectroscopy in Astrophysics (Springer, Berlin, 2007)
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