ESPRESSO sees the light at the end of the “tunnel”
On 25 September 2016, for the first time the light of a star observed from the UT4 telescope located at the European Southern Observatory ESO/VLT (Paranal) reached the Coudé Combined Laboratory where ESPRESSO spectrograph will be placed in 2017. This remarkable success has been obtained also with the important contribution of Italian researchers from the Italian National Institute for Astrophysics (INAF), in particular from of Trieste and Milan Observatories.
ESPRESSO (): the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations is a fiber-fed, cross-dispersed echelle spectrograph characterized by ultra-high stability, precision and resolution currently in the final integration phase at the Astronomical Observatory of Geneva (OGE). Once installed at the European Southern Observatory in Paranal it will produce spectra with a precision never reached before by similar spectrographs; a significant impact in several areas of astronomical research is thus expected, in particular for the detection and characterization of exo-planets and for the detection of possible variability (or not) of fundamental constants. ESPRESSO is built by a consortium with partners in Switzerland, Portugal, Spain, besides ESO, and Italy with a significant participation of researchers from INAF, and in particular from the Observatories of Trieste and Milan (PI: F. Pepe - OGE, co-PI: S. Cristiani, project scientist: P. Molaro, INAF-OATs).
In order to fulfill its demanding scientific cases, ESPRESSO will be installed in the Coudé Combined Laboratory (CCL) where it could use either the light of any of the UTs or incoherently the combined light of all the four UTs (the light path distance is of the order of up to 70 m, depending on the telescope) and in this case mimic a telescope of aperture of 16 m.
The four optical paths that relay the light from the telescopes to the CCL are named “coudé trains”. Each of them hosts nine different large optical elements of superb quality, installed inside tunnels covering a distance of about 60 m, capable to deliver the light with minimum aberrations and maximum efficiency.
At the beginning of September the installation of the opto-mechanics and the successive optical alignment took place on telescope UT4 of the VLT. The alignment phase was extremely delicate due to the very high precision requirements in positioning over the long distance. Simultaneously in the CCL one of the ESPRESSO’s front-end was installed. The front-end is the part of the spectrograph that collects light from the coudé trains and feeds the fibers of the spectrograph once installed. All the four front-ends needed both for the opto-mechanics as well as for the control electronics and software are responsibility of INAF (Milan and Trieste, respectively).
The first light of the above described system composed by the coudé train on UT4 and by one of the ESPRESSO front-end achieved the first light successfully during the night of 25 September. The great effort put in the design, manufacturing, installation and alignment of this system was well paid off from the excellent preliminary results obtained already with first telescope pointings. The delivered image of a bright star measured at the focus had a FWHM of less than 0.5 arcsec. The quality of the system is also testified by the uniformity of the image quality across the field of view of 17 arcsec, and the stability of the light beam which are fundamental prerequisites to fulfil the ambitious scientific cases for ESPRESSO.
With the successful first light achievement the green light is expected from ESO to install the light paths for the remaining three UTs in order to allow ESPRESSO to get light from celestial objects, or as should be properly said, to see the light at the end of the tunnels.
The very first light after one of the first telescope presets imaged on the two front-end detectors. On the left, the star image with a FWHM around 0.5 arcsec; the goodness of pointing/alignment could be seen looking at the fiber hole very close to the star position (circular dark hole slightly on top on the left). On the right the image of the pupil; the secondary mirror and the spider shadows are very sharp, testifying the excellent image quality in the pupil as well.
|Schema of the coudé-train|
 The Consortium is formed by the following institutes:
Observatoire de Geneva and University of Bern, Switzerland
INAF-Trieste and Merate, Italy
Instituto de Astrofísica de Canarias, Spain
Universidade do Porto and Lisboa, Portugal