Free Astronomy Magazine March-April 2018

MARCH-APRIL 2018 l i g h t 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 un- derground structure of the moun- taintop from the start. A system of mirrors, prisms and lenses transmits the light from each VLT Unit Tele- scope 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, in- creasing its light-gathering power, or alternatively receive light from any one of the Unit Telescopes inde- pendently, allowing for more flexi- ble 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 Interferom- eter for the study of extremely fine detail in compar- atively bright ob- jects. Project Scientist Paolo Molaro com- ments: “This im- pressive milestone is the culmination of work by a large team of scientists and engi- neers over many years. It is wonderful to see ESPRESSO working with all four Unit Telescopes and I look forward to the exciting sci- ence results to come.” Feeding the combined light into a single instru- ment will give astronomers access to information never previously avail- able. 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 re- peatability, and now 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 Tele- scope, will offer in a few years,” concludes ESO’s Director General, Xavier Barcons. -metre telescope ! T his picture shows in highly simplified form how the light collected by all four VLT Unit Telescopes is combined in the ESPRESSO instrument, located under the VLT platform. [ESO/L. Calçada]