ESPRESSO - An Echelle SPectrograph for Rocky Exoplanets Search and Stable Spectroscopic Observations,
The Messenger, Volume 153, pp. 6 (2013)

ESPRESSO is the next generation European exoplanet hunter, combining the efficiency of a modern echelle spectrograph with state-of-the-art 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. ESPRESSO will reach the 10 cm s^-1 level in radial velocity precision and will also 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 several innovative design solutions while ensuring the technical heritage of HARPS.

Figure 1 | Detectability of planets orbiting a 0.8 M_ʘ star (red solid line) and a 1.0 M_ʘ star (green solid line) in the mass vs. semi-major axis plane expected for ESPRESSO. The detectability curves have been calculated assuming a radial velocity semi-amplitude of 10 cm s^–1 (for the 1.0 M_ʘ star) and 1 m s^–1 (for the 0.8 M_ʘ star), zero eccentricity, and sin i = 1. Known RV planets of solar-type stars are plotted as open circles, and the planets of the Solar System (solid circles) are labelled. The "habitable zones" of 0.8–1.2 M_ʘ and 0.2–0.3 M_ʘ stars are indicated by the blue and pink dotted areas, respectively. These are regions where rocky planets with a mass in the interval 0.1–10 M_⊕ can retain liquid water on their surface.

Spectroscopic data has become a cornerstone for research in astronomy and there is a large and growing demand for stable and precise spectrographs. Such observational data is mandatory for many fields of astronomy, with a special focus on the search for exoplanets, which demands ultra-stable instrumentation, and the search for variability of physical constants.

The need for such high-quality spectroscopic data has been widely recognised by the European Southern Observatory (ESO) and the European Space Agency and has led ESO to support the development of a new-generation spectrograph for the VLT. Thus, ESPRESSO, the Echelle SPectrograph for Rocky Exoplanets Search and Stable Spectroscopic Observations, was developed for ESO by a consortium of academic and research institutions from Italy, Portugal, Spain and Switzerland, and will be built by the ESO instrumentation division.

The main goals that drove the need for ESPRESSO are the measurement of high-precision radial velocities to search for rocky planets (Fig. 1), the measurement of the variation of physical constants and the analysis of the chemical composition of stars in nearby galaxies. Only by using a spectrograph with the precision and stability of ESPRESSO can these goals be achieved.

In fact, ESPRESSO, being a fibre-fed, cross-dispersed, high-resolution echelle spectrograph, represents an order of magnitude improvement in precision when compared to its predecessor HARPS, currently installed on the 3.6 m telescope in La Silla. The light captured by the VLT telescopes will be routed to ESPRESSO – located in the combined coudé laboratory – through the coudé train optical system. The front end unit allows the spectrograph to use the light of any single unit telescope (UT) or from up to all 4 UTs simultaneously. This will allow ESPRESSO to work in 3 different modes: high resolution using light from a single UT, ultra-high resolution using light from a single UT or mid resolution using light from up to 4 UT (Table 1).

In order to optimize the sensitivity to different wavelengths, ESPRESSO has two different optical arms – a “red” and a “blue” arm – each with its own dedicated monolith detector. ESPRESSO is located within a vacuum vessel in a multi-shell thermal control system. This structure ensures temperature is stable down to the level of the mK, thus providing a critical contribution to the high stability achieved by this spectrograph.

ESPRESSO has passed the final design review in May 2013 and is currently in the manufacturing phase. The instrument will be integrated in Europe in early 2015 and will be installed in Chile in the following year. ESPRESSO is expected to start its scientific observations program by the end of 2016.

Besides its role in the leadership of this project, CAUP is also responsible for the development of the data reduction and analysis software, as well as of the Coudé Train control software. Furthermore, CAUP also participates in the global scientific planning of ESPRESSO. The Portuguese contribution to the project involves the design and construction of one of the key components of the instrument: the Coudé Train. This component will collect the light from the four 8.2-m Unit Telescopes that compose the VLT, and transform it in the equivalent of a 16-m telescope, several years ahead of the E-ELT.

1. PI, in representation of the Swiss team of the ESPRESSO Consortium
2. Co-PI, in representation of the Portuguese team of the ESPRESSO Consortium
3. Co-PI, in representation of the Spanish team of the ESPRESSO Consortium
4. Co-PI, in representation of the Italian team of the ESPRESSO Consortium
5. Co-PI, in representation of the ESO team of the ESPRESSO Consortium