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ESPRESSO is the next generation Euro- pean exoplanet hunter, combining the ef ciency of a modern echelle spectro- graph with extreme radial velocity and spectroscopic precision. ESPRESSO will be installed in the Combined Coudé Laboratory of the VLT and linked to the four Unit Telescopes (UT) through optical coudé trains, operated either with a single UT or with up to four UTs for 1.5 magnitude gain. The instrumental radial velocity precision will reach the 10 cm s–1 level and ESPRESSO will achieve a gain of two magnitudes with respect to its predecessor HARPS.
This is the first VLT instrument using the incoherent combination of light from four telescopes and, together with the extreme precision requirements, calls for many innovative design solutions while ensuring the technical heritage of HARPS.
The main scientific drivers for ESPRESSO are the search and characterisation of rocky exoplanets in the habitable zone of quiet, nearby G to M dwarf stars and the analysis of the variability of fundamental physical constants. As an ultrastable highresolution spectrograph however, ESPRESSO will allow new frontiers to be explored in most domains of astrophysics. The instrument has been installed at the Paranal Observatory in 2017 and will begin official operations by October 2018.
High resolution spectroscopy has always been at the heart of astrophysics. It provides the data that bring physical insight into the behaviour of stars, galaxies, interstellar and intergalactic media. Correspondingly, high-resolution spectrographs have always been in high demand at major observatories, see, e.g., UVES at the VLT or HIRES at the Keck Telescope. As telescope apertures become larger, the capabilities of high-resolution spectrographs extend to fainter and fainter objects. Besides this increase in photon-collecting power, another aspect has emerged in recent years: the power of high-precision spectroscopy. In many applications there is the need for highly repeatable observations over long time scales where instrumental effects must be completely removed, or at least minimised. For instance, this is the case for radial velocity (RV) measurements, or, more generally, for the determination of the positions and shapes of spectral lines. In this respect, the HARPS spectrograph at the ESO 3.6-metre telescope (Mayor et al., 2003) has been a pioneering instrument. It has been widely recognised in the European astronomical community that a similar instrument on the VLT would be necessary.
The need for a groundbased followup facility capable of high RV precision was stressed in the ESO–ESA working group report on extrasolar planets (Perryman et al., 2005). The research area “terrestrial planets in the habitable zone” is one of the main scientific topics for the next few decades in astronomy, and one of the main science drivers for the new generation of extremely large telescopes (ELTs). For instance, the ESO–ESA working group report calls for “high-precision radial-velocity instrumentation for the follow-up of astrometric and transit detections, to ensure the detection of a planet by a second independent method, and to determine its true mass. For Jupiter-mass planets, existing instrumentation may be technically adequate, but observing time inadequate; for Earth-mass candidates, special-purpose instrumentation (like HARPS) on a large telescope would be required.” (Perryman et al., 2005, p. 63). The same concept is reiterated in the first recommendation: “Support experiments to im prove RV mass detection limits, e.g., based on experience from HARPS, down to those imposed by stellar surface phenomena” (Perryman et al., 2005, p. 72).
Do the fundamental constants vary? This is one of the six big open questions in cosmology as listed in the ESA–ESO working group report for fundamental cosmology (Peacock et al., 2006). In the executive summary, the document states: “Quasar spectroscopy also offers the possibility of better constraints on any time variation of dimensionless atomic parameters such as the fine-structure constant α and the proton-to-electron mass ratio. Presently there exist controversial claims of evidence for variations in α, which potentially relate to the dynamics of dark energy. It is essential to validate these claims with a wider range of targets and atomic tracers.” This goal can only be reached with improved spectroscopic capabilities.
In this context the ESO Scientific Technical Committee (STC) recommended, at its 67th meeting in October 2007, the development of additional second generation VLT instruments, and its detailed proposal was endorsed by the ESO Council at its 111th meeting in December 2007. Among the recommended instruments, a high-resolution, ultra-stable spectrograph for the VLT combined coudé focus arose as a cornerstone to complete the current second generation VLT instrument suite. In March 2007, following these recommendations, ESO issued a call for proposals, open to Member State institutes or consortia, to carry out the Phase A study for such an instrument. The submitted proposal was accepted by ESO and the ESPRESSO consortium was selected to carry out the project for the construction of this spectrograph. The main scientfiic drivers for this project were de ned by ESO as follows:
1. Measure high-precision RV to search for rocky planets.
2. Measure the variation of physical constants.
3. Analyse the chemical composition of stars in nearby galaxies.
The official project kick-off was held in February 2011. The design phase lasted about 2.5 years and ended with the final design review (FDR) in May 2013. The procurement of components and manufacturing of subsystems lasted longer than planned due to manufacturing difficulties. Early 2015 the first subsystems were ready for integration in Europe, but Provisional Acceptance Europe of the instrument (the end integration and Verification) could only be held in June 2017. The transfer of the instrument to Paranal, installation took place in August-November 2017. The commission time cover the southern summer 2017-2018. Acceptance Paranal is planned to take place mid-2018.
ESPRESSO is a fibre-fed, cross-dispersed, high-resolution, echelle spectrograph. The telescope light is fed to the instrument via a so-called ‘Coudé-Train’ optical system and within optical fibres. ESPRESSO is located in the Combined-Coudé Laboratory (incoherent focus) where a front-end unit can combine the light from up to 4 Unit Telescopes (UT) of the VLT. The target and sky light enter the instrument simultaneously through two separate fibres, which form together the ‘slit’ of the spectrograph.
Several optical ‘tricks’ have been used to obtain high spectral resolution and efficiency despite the large size of the telescope and the 1 arcscec sky aperture of the instrument. At the spectrograph entrance the Anamorphic Pupil Slicing Unit (APSU) shapes the beam in order to compress it in cross-dispersion and splits in two smaller beams, while superimposing them on the echelle grating to minimize its size. The rectangular white pupil is then re-imaged and compressed. Given the wide spectral range, a dichroic beam splitter separates the beam in a blue and a red arm, which in turn allows to optimizing each arm for image quality and optical efficiency. The cross-disperser has the function of separating the dispersed spectrum in all its spectral orders. In addition, an anamorphism is re-introduced to make the pupil square and to compress the order height such that the inter-order space and the SNR per pixel are both maximized. Both functions are accomplished using Volume Phase Holographic Gratings (VPHGs) mounted on prisms. Finally, two optimised camera lens systems image the full spectrum from 380 nm to 780 nm on two large 92 mm x 92 mm CCDs with 10-um pixels.
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