VLT's Interferometer Goes Deeper
6 June 2011
On the nights of 15–17 May 2011, astronomers successfully applied a new observing technique on ESO’s Very Large Telescope Interferometer (VLTI). The new procedure is called “blind observation mode” and will allow interferometric observations to be made of targets significantly fainter than before. It has already produced detailed observations of the spectrum of a quasar, 10 times fainter than the best currently known results. The result was obtained by combining the light from three of the VLT’s 8.2-metre Unit Telescopes in the AMBER  beam combiner.
The ground-breaking technique, developed by a team of astronomers led by Romain Petrov from the Laboratoire Fizeau (Université de Nice, Observatoire de la Côte d’Azur and CNRS, France), allows them to observe targets that would ordinarily be far too faint to be detected by an interferometer.
They do this by first observing a bright star near the faint target, which allows them to calibrate the interferometer’s systems properly. When the beams of light from the individual telescopes are successfully combined, a characteristic pattern of bright and dark lines is seen, known as fringes. Then, the astronomers quickly switch to observing the faint target, which is said to be observed blindly because its fringes are too faint to be seen. However, the team can make their observations, knowing that the interferometer’s systems remain correctly calibrated from the earlier bright star. By combining multiple apparently empty exposures of the faint target, its fringes can be successfully revealed.
The quasar 3C 273 — an active galaxy with a supermassive black hole in its core — was the first target successfully observed with this new procedure. The team’s observations allow them to study the size, structure, and movement of the innermost material surrounding the central black hole.
The new blind observation technique has a huge potential and, by pushing the capabilities of current interferometers to fainter limits, is expected to bring a number of other previously unobservable targets within reach.
 AMBER, the Astronomical Multi-BEam combineR, is an instrument on the VLTI used to combine the light coming from two or three of the VLTI’s telescopes. In this case, AMBER was used to combine beams from Unit Telescopes UT1, UT2, and UT4.
The “blind observation mode” work was done by Romain Petrov and Florentin Millour (Laboratoire Fizeau, UNS/OCA/CNRS, Nice, France), Gerd Weigelt (MPIfR, Bonn, Germany), and Alessandro Marconi (University of Firenze and INAF-Osservatorio di Arcetri, Italy) using AMBER consortium Guaranteed Time Observations, with the help of Antoine Merand from ESO/Paranal.
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