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The Instrument

GRAVITY is a second-generation instrument for the Very Large Telescope Interferometer (VLTI). It is designed to take spectra of objects in the near-infrared K band at very high spatial resolution, up to the equivalent of a 200m diameter telescope. 

It is a complex system consisting of adaptive-optics, laser metrology, acquisition camera, piston control, four beam interferometric combiner, fringe tracker, spectrograph and a near-infrared detectors.  It was developed by a consortium of German, French and Portuguese institutes together with ESO.

The I. Physics Institute was responsible for the development of the two spectrographs which take the light from the beam combiners, collimate it through the dispersive grisms and polarimetric Wollaston prism, finally focusing the spectra onto the detectors. The spectrographs also need to receive the light of the metrology laser and guide it into the beam combiner, for propagation back through the rest of the optical system.

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  • The telescopes of the VLTI interferometer. In the background are the four large VLT telescopes used for observations of faint targets, such as the galactic centre and quasars. In the foreground are the smaller auxiliary telescopes used for brighter targets, and when the VLT telescopes are needed for single telescope science. The squat, sunlit, building contains the VLTI control room and lab. (Credit: ESO)
  • The GRAVITY cryostat opened. This part of the instrument consists of the acquisition camera, piston control system, beam combiner, the two I. Physics Institute developed spectrographs (in foil, on the right) and the detector systems. (Credit: ESO)
  • A CAD view of the optical train of the GRAVITY science spectrograph.