With the first light of the ESPRESSO spectrograph using the four-unit telescope mode of the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT), the device has become a single giant telescope.

After extensive preparations by the ESPRESSO consortium led by the Astronomical Observatory at the University of Geneva, with the participation of research centers from Italy, Portugal, Spain and Switzerland, and ESO staff, ESO's Director General Xavier Barcons initiated this historic astronomical observation with the push of a button in the control room.

"ESO has realized a dream that dates back to the time when the VLT was conceived in the 1980s: combining the light of all four unit telescopes on Cerro Paranal to feed a single instrument,” said Gaspare Lo Curto, ESPRESSO instrument scientist at ESO.

When all four 8.2-m unit telescopes combine their light-collecting power to feed a single instrument, the VLT effectively becomes the largest optical telescope in the world, in terms of collecting area.

ESPRESSO goals include the discovery and characterization of Earth-like planets and the search for possible variability of the fundamental constants of physics. The latter experiments require the observation of distant and faint quasars, and this goal will benefit the most from combining the light from all four unit telescopes. Both goals rely on the ultrahigh stability of the instrument and an extremely stable reference light source.

A system of mirrors, prisms and lenses transmits the light from each VLT unit telescope to the ESPRESSO spectrograph up to 69 m away. Thanks to these complex optics, ESPRESSO can either collect the light from up to all four unit telescopes together, increasing its light-gathering power, or receive light from any one of the unit telescopes independently, allowing for more flexible usage of observing time. ESPRESSO was specially developed to exploit this infrastructure.

Light from the four unit telescopes is routinely brought together in the VLT interferometer for the study of extremely fine detail in comparatively bright objects. But interferometry, which combines the beams coherently, cannot exploit the huge light-gathering potential of the combined telescopes to study faint objects.

"This impressive milestone is the culmination of work by a large team of scientists and engineers over many years,” said Paolo Molaro, project scientist at ESO. “It is wonderful to see ESPRESSO working with all four unit telescopes, and I look forward to the exciting science results to come."

Feeding the combined light into a single instrument will give astronomers access to information never previously available. This new facility makes use of novel concepts, such as wavelength calibration aided by a laser- frequency comb, providing unprecedented precision and repeatability. It now provides the capability to join together the light-collecting power of the four individual unit telescopes.

"ESPRESSO working with all four unit telescopes gives us an enticing foretaste of what the next generation of telescopes, such as ESO's Extremely Large Telescope, will offer in a few years," Barcons said.

ESO is an intergovernmental astronomy organization and ground-based astronomical observatory supported by the countries of Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the U.K., along with the host state of Chile.