The European Southern Observatory ESO has commissioned the espresso spectrograph, a special instrument with which the Very Large Telescope in Chile can bundle the light from its four 8.2-metre telescopes.
Using the espresso spectrograph, the light from the four 8.2-metre Unit telescopes can be combined, making the Very Large Telescope the largest optical telescope in the world in terms of light-receiving surface. The VLT is therefore comparable to a single telescope with an aperture of 16 meters. During the design of the VLT it was already the intention to have the four telescopes work together as one large telescope.
The system of combining the light from the four auxiliary telescopes works through a system of mirrors, prisms and lenses. These optical components bring the light from each individual telescope to the spectrograph over a distance of up to 69 meters. This instrument can combine the light from two or more of the Unit telescopes to either increase the light output of the VLT or collect the light from the individual telescopes. According to ESO, this allows a more flexible use of the observation time.
An interferometer was already used at the VLT to combine the light from the different telescopes. That makes it possible to see details of a space object as if it were observed with a mirror hundreds of meters in diameter. The distance between two telescopes is decisive here; this forms, as it were, the diameter of the imaginary telescope. In principle, the four telescopes together can equal the resolution of a telescope with a diameter of 200 meters. With the espresso instrument, the combined light is fed to a single instrument, providing information that was previously unavailable.
The special instrument, developed by the University of Geneva and research centers in Italy, Portugal, Spain and Switzerland, among others, is specifically intended to enable even better discovery and characterization of Earth-like planets, and to search for possible variations in ‘fundamental physical constants’. ‘. The latter could include studying distant and faint quasars. These are supermassive black holes in the center of a galaxy, which emit a lot of radiation.
According to ESO’s director, Xavier Barcons, this technique offers a foretaste of what the next generation of telescopes will bring to science. In a few years, for example, ESO’s Extremely Large Telescope will be ready. The contract to build this huge European telescope was signed in 2016. The telescope should be operational in 2024 and will have a main mirror with a diameter of 39 meters, which mirror is made up of 798 hexagonal mirror segments.