The European Southern Observatory’s Very Large Telescope (VLT) has finally reached its original design goal by functioning as 16-meter telescope for first time.
ESO’s Director General Xavier Barcons did the honors by pushing a button in the control room enabling all four 8.2-metre Unit Telescopes to combine their light-collecting power to feed a single instrument. Because of this VLT has effectively become the largest optical telescope in the world in terms of collecting area.
ESPRESSO has two main scientific goals – discovery and characterisation of Earth-like planets and the search for possible variability of the fundamental constants of physics. The second scientific goal calls for observation of distant and faint quasars, and this science goal will benefit the most from combining the light from all four Unit Telescopes in ESPRESSO. Both rely on the ultra-high stability of the instrument and an extremely stable reference light source.
Due to the complexity involved, the combination of light from all four Unit Telescopes in this way, at what is known as an “incoherent focus”, had not been implemented until now. However, space for it was built into the telescopes and the underground structure of the mountaintop from the start.
A system of mirrors, prisms and lenses transmits the light from each VLT Unit Telescope to the ESPRESSO spectrograph up to 69 metres 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 alternatively 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.
Feeding the combined light into a single instrument will give astronomers access to information never previously available. This new facility is a game changer for astronomy with high-resolution spectrographs. It makes use of novel concepts, such as wavelength calibration aided by a laser frequency comb, providing unprecedented precision and repeatability, and now the capability to join together the light-collecting power of the four individual Unit Telescopes.