10 The Messenger 170 – December 2017 Phase referencing optical interferometry for the VLTI As part of GRAVITY, each of the four 8.2-metre Unit Telescopes (UT) has been equipped with a Coudé Infrared Adaptive Optics (CIAO) system, which corrects atmospheric perturbations and stably injects the light from two adjacent astro- physical objects into optical fbres. Two fbres per telescope feed two integrated optics beam combiners: one for each object. Each beam combiner pairwise combines the telescopes and samples the resulting six fringe patterns at four phase- shifted locations. Those 24 outputs are dispersed by a spectrometer and imaged onto a detector. One of the two beam combiners serves as a fringe-tracker. It is equipped with a low-resolution spec- trometer and a fast camera optimised to measure the fringe phase at kHz fre- quency. The fringe-tracker measures and corrects atmospheric piston perturbations in real time, which permits long integra- tion times on the second beam combiner. The active stabilisation of the science channel allows the integration time to be increased from a few milliseconds (atmospheric coherence time) to hun- dreds of seconds. The resulting leap in sensitivity opens up a wide range of applications, from very faint targets at low spectral resolution to moderately bright targets at high spectral resolution. Furthermore, GRAVITY provides dual- feld astrometry by precisely measuring the optical delay between two distinct objects with a dedicated metrology sys- tem. This yields object separations with an exquisite accuracy of a few tens of μas. Both single-feld (where a beam splitter injects the light from a single object into the two fbres) and dual-feld modes are offered to the community, with a spectral resolution up to 4000 and a limiting mag- nitude of K = 8 in single-feld mode using the 1.8-metre Auxiliary Telescopes (ATs) and K = 10 on the 8-metre UTs depending on weather conditions. In dual-feld mode, the offered limiting magnitude is 0.5 mag- nitudes fainter on the fringe-tracker, and 3.5 magnitudes on the science channel. Dual-feld astrometry is still under com- missioning. The frst observations show residuals as low as 50 μas when follow- ing objects over several months. GRAVITY Collaboration: Telescopes and Instrumentation First Light for GRAVITY: A New Era for Optical Interferometry Roberto Abuter 8 , Matteo Accardo 8 , António Amorim 6 , Narsireddy Anugu 7 , Gerardo Ávila 8 , Myriam Benisty 5 , Jean-Philippe Berger 5 , Nicolas Blind 9 , Henri Bonnet 8 , Pierre Bourget 8 , Wolfgang Brandner 3 , Roland Brast 8 , Alexander Buron 1 , Frédéric Cassaing 10 , Frédéric Chapron 2 , Élodie Choquet 2 , Yann Clénet 2 , Claude Collin 2 , Vincent Coudé du ForestoI 2 , Willem-Jan de Wit 8 , Tim de Zeeuw 8, 14 , Casey Deen 1 , Françoise Delplancke-Ströbele 8 , Roderick Dembet 8 , Frédéric Derie 8 , Jason Dexter 1 , Gilles Duvert 5 , Monica Ebert 3 , Andreas Eckart 4, 13 , Frank Eisenhauer 1 , Michael Esselborn 8 , Pierre Fédou 2 , Gert Finger 8 , Paulo Garcia 7 , Cesar Enrique Garcia Dabo 8 , Rebeca Garcia Lopez 3 , Feng Gao 1 , Éric Gendron 2 , Reinhard Genzel 1, 15 , Stefan Gillessen 1 , Frédéric Gonté 8 , Paulo Gordo 6 , Marion Grould 2 , Ulrich Grözinger 3 , Sylvain Guieu 5, 8 , Pierre Haguenauer 8 , Oliver Hans 1 , Xavier Haubois 8 , Marcus Haug 1, 8 , Frank Haußmann 1 , Thomas Henning 3 , Stefan Hippler 3 , Matthew Horrobin 4 , Armin Huber 3 , Zoltan Hubert 2 , Norbert Hubin 8 , Christian A. Hummel 8 , Gerd Jakob 8 , Lieselotte Jochum 8 , Laurent Jocou 5 , Martina Karl 1 , Andreas Kaufer 8 , Stefan Kellner 1, 13 , Sarah Kendrew 3, 11 , Lothar Kern 8 , Pierre Kervella 2, 12 , Mario Kiekebusch 8 , Ralf Klein 3 , Johan Kolb 8 , Martin Kulas 3 , Sylvestre Lacour 2 , Vincent Lapeyrère 2 , Bernard Lazareff 5 , Jean- Baptiste Le Bouquin 5 , Pierre Léna 2 , Rainer Lenzen 3 , Samuel Lévêque 8 , Magdalena Lippa 1 , Yves Magnard 5 , Leander Mehrgan 8 , Marcus Mellein 3 , Antoine Mérand 8 , Javier Moreno-Ventas 3 , Thibaut Moulin 5 , Eric Müller 3, 8 , Friedrich Müller 3 , Udo Neumann 3 , Sylvain Oberti 8 , Thomas Ott 1 , Laurent Pallanca 8 , Johana Panduro 3 , Luca Pasquini 8 , Thibaut Paumard 2 , Isabelle Percheron 8 , Karine Perraut 5 , Guy Perrin 2 , Pierre-Olivier Petrucci 5 , Andreas Pfüger 1 , Oliver Pfuhl 1 , Thanh Phan Duc 8 , Philipp M. Plewa 1 , Dan Popovic 8 , Sebastian Rabien 1 , Andrés Ramírez 8 , Jose Ramos 3 , Christian Rau 1 , Miguel Riquelme 8 , Gustavo Rodríguez-Coira 2 , Ralf-Rainer Rohloff 3 , Alejandra Rosales 1 , Gérard Rousset 2 , Joel Sanchez- Bermudez 3 , Sylvia Scheithauer 3 , Markus Schöller 8 , Nicolas Schuhler 8 , Jason Spyromilio 8 , Odele Straub 2 , Christian Straubmeier 4 , Eckhard Sturm 1 , Marcos Suarez 8 , Konrad R. W. Tristram 8 , Noel Ventura 5 , Fédéric Vincent 2 , Idel Waisberg 1 , Imke Wank 4 , Felix Widmann 1 , Ekkehard Wieprecht 1 , Michael Wiest 4 , Erich Wiezorrek 1 , Markus Wittkowski 8 , Julien Woillez 8 , Burkhard Wolff 8 , Senol Yazici 1, 4 , Denis Ziegler 2 , Gérard Zins 8 1 Max Planck Institute for Extraterrestrial Physics, Garching, Germany; 2 LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Université Paris 6, Université Paris Diderot, Sorbonne Paris Cité, France; 3 Max Planck Institute for Astronomy, Heidelberg, Germany; 4 I. Physikalisches Institut, University of Cologne, Germany; 5 Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, IPAG, France; 6 CENTRA — Universidade de Lisboa — Faculdade de Ciências, Portugal; 7 CENTRA — Universidade do Porto — Faculdade de Engenharia, Portugal; 8 ESO; 9 Observatoire de Genève, Université de Genève, Switzerland; 10 ONERA, The French Aerospace Lab, Châtillon, France; 11 European Space Agency, Space Telescope Science Institute, Baltimore, USA; 12 Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Santiago, Chile; 13 Max Planck Institute for Radio Astronomy, Bonn, Germany; 14 Leiden Observatory, The Netherlands; 15 Department of Physics, University of California, Berkeley, USA With the arrival of the second generation instrument GRAVITY, the Very Large Telescope Interferometer (VLTI) has entered a new era of optical interferom- etry. This instrument pushes the limits of accuracy and sensitivity by orders of magnitude. GRAVITY has achieved phase-referenced imaging at approxi- mately milliarcsecond (mas) resolution and down to ~ 100-microarcsecond astrometry on objects that are several hundred times fainter than previously observable. The cutting-edge design presented in Eisenhauer et al. (2011) has become reality. This article sketches out the basic principles of the instrument design and illustrates its performance with key science results obtained dur- ing commissioning: phase-tracking on stars with K ~ 10 mag, phase-referenced interferometry of objects fainter than K ≳ 17 mag, minute-long coherent inte- grations, a visibility accuracy of better than 0.25 %, and spectro-differential phase and closure phase accuracy better than 0.5 degrees, corresponding to a differential astrometric precision of a few microarcseconds (μas). GRAVITY was developed in a collaboration between the Max Planck Institute for Extra- terrestrial Physics, LESIA of Observatoire de Paris/CNRS/UPMC/Université Paris Diderot and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, CENTRA–Centro Multidisciplinar de Astrofísica (Lisbon and Porto) and ESO. GRAVITY was shipped to Paranal in July 2015 (Figures 1 and 2). After roughly one year of installation and commission- ing, the instrument was offered to the scientifc community for the frst time in October 2016. The observations we report here include objects and results that were out of reach for any previous interferometric instrument. Most notable are observations of the Galactic Centre black hole Sagittarius A* (Sgr A*) and the nearby star S2 (Figure 3); the micro- quasar SS 433; and 50-microarcseconds accuracy astrometry of the M-dwarf binary system GJ 65. More details about GRAVITY and early commissioning results can be found in GRAVITY Collabo- ration (2017 a, b, c and d). DOI: 10.18727/0722-6691/5048