16 The Messenger 143 – March 2011 Telescopes and Instrumentation GRAVITY is the second generation Very Large Telescope Interferometer instru- ment for precision narrow-angle as- trometry and interferometric imaging. With its ibrefed integrated optics, wavefront sensors, fringe tracker, beam stabilisation and a novel metrology concept, GRAVITY will push the sensi- tivity and accuracy of astrometry and interferometric imaging far beyond what is offered today. Providing precision astrometry of order 10 microarcseconds, and imaging with 4-milliarcsecond resolution, GRAVITY will revolutionise dynamical measurements of celestial objects: it will probe physics close to the event horizon of the Galactic Centre black hole; unambiguously detect and measure the masses of black holes in massive star clusters throughout the Milky Way; uncover the details of mass accretion and jets in young stellar objects and active galactic nuclei; and probe the motion of binary stars, exo- planets and young stellar discs. The instrument capabilities of GRAVITY are outlined and the science opportunities that will open up are summarised. Fundamental measurements over a wide range of ields in astrophysics Much as long-baseline radio interfer- ometry has tone, GRAVITY infrared (IR) astrometry, with an accuracy of order 10 microarcseconds and phase-referenced imaging with 4-milliarcsecond resolution, will bring a number of key advances (Eisenhauer et al., 2008). GRAVITY will carry out the ultimate empirical test to show whether or not the Galactic Centre harbours a black hole (BH) of four million solar masses and will inally decide if the near-infrared lares from Sgr A* origi- nate from individual hot spots close to the last stable orbit, from statistical luc- tuations in the inner accretion zone or from a jet. If the current hot-spot interpre- tation of the near-infrared (NIR) lares is correct, GRAVITY has the potential to directly determine the spacetime metric around this BH. GRAVITY may even be able to test the theory of general rela- tivity in the presently unexplored strong ield limit. GRAVITY will also be able to unambiguously detect intermediate mass BHs, if they exist. It will dynamically measure the masses of supermassive Frank Eisenhauer 1 Guy Perrin 2, 10 Wolfgang Brandner 3 Christian Straubmeier 4 Karine Perraut 5 António Amorim 6 Markus Schöller 9 Stefan Gillessen 1 Pierre Kervella 2, 10 Myriam Benisty 3 Constanza Araujo-Hauck 4 Laurent Jocou 5 Jorge Lima 6 Gerd Jakob 9 Marcus Haug 1 Yann Clénet 2, 10 Thomas Henning 3 Andreas Eckart 4 Jean-Philippe Berger 5,9 Paulo Garcia 6 Roberto Abuter 9 Stefan Kellner 1 Thibaut Paumard 2, 10 Stefan Hippler 3 Sebastian Fischer 4 Thibaut Moulin 5 Jaime Villate 6 Gerardo Avila 9 Alexander Gräter 1 Sylvestre Lacour 2, 10 Armin Huber 3 Michael Wiest 4 Axelle Nolot 5 Pedro Carvas 6 Reinhold Dorn 9 Oliver Pfuhl 1 Eric Gendron 2, 10 Sarah Kendrew 3 Senol Yazici 4 Sonia Anton 6,8 Yves Jung 9 Markus Thiel 1 Élodie Choquet 2, 10 Ralf Klein 3 Paula Teixeira 6,9 Philippe Gitton 9 David Moch 1 Frédéric Vincent 2, 10 Natalia Kudryavtseva 3 Stefan Ströbele 9 Eckhard Sturm 1 Pierre Fédou 2, 10 Rainer Lenzen 3 Paul Jolley 9 Clemens Kister 1 Vincent Lapeyrère 2, 10 Vianak Naranjo 3 Christian Lucuix 9 Reiner Hofmann 1 Frédéric Chapron 2, 10 Udo Neumann 3 Leander Mehrgan 9 Oliver Hans 1 Gérard Rousset 2, 10 Jose Ramos 3 Marcos Suarez 9 Reinhard Lederer 1 Jean-Michel Reess 2, 10 Ralf-Rainer Rohloff 3 Pierre Haguenauer 9 Hendrik Bartko 1 Arnaud Sevin 2, 10 Karl Wagner 3 Jean-Louis Lizon 9 Sebastian Rabien 1 Claude Collin 2, 10 Gert Finger 9 Richard Davies 1 Daniel Rouan 2, 10 Markus Wittkowski 9 Katie Dodds-Eden 1 Denis Ziegler 2, 10 Frédéric Cassaing 7, 10 Henri Bonnet 9 Mark Casali 9 Reinhard Genzel 1 Pierre Lena 2 1 Max-Planck Institute for Extraterrestrial Physics, Garching, Germany 2 LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, Meudon, France 3 Max-Planck Institute for Astronomy, Heidelberg, Germany 4 Physikalisches Institut, University of Cologne, Germany 5 UJF–Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble, France 6 Laboratório de Sistemas, Instrumen- tação e Modelação em Ciências e Tecnologias do Ambiente e do Espaço (SIM), Lisbon and Porto, Portugal 7 ONERA, Optics Department (DOTA), Châtillon, France 8 Centro de Investigação em Ciências Geo-Espaciais, Porto, Portugal 9 ESO 10 Groupement d’Intérêt Scienti ique PHASE (Partenariat Haute résolution Angulaire Sol Espace) between ONERA, Observatoire de Paris, CNRS and Université Paris Diderot GRAVITY: Observing the Universe in Motion