First light for the GRAVITY+ Adaptive Optics: Extreme adaptive optics for the Very Large Telescope Interferometer

¹ Max Planck Institute for extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
² LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ 1 st Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias, 4200-465 Porto, Portugal
⁷ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁸ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁹ Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹⁰ CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹¹ Department of Physics, Technical University Munich, James-Franck-StraÃe 1, 85748 Garching, Germany
¹² Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA
¹⁴ School of Physics & Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
¹⁵ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
¹⁶ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹⁷ School of Physics, University College Dublin, Dublin 4, Belfield, Ireland
¹⁸ Instituto de Astronomia, Universidad Nacional Autónoma de México, Apdo. Postal 70264, Ciudad de Mexico 04510, Mexico
¹⁹ Laboratoire Lagrange, Univ. Côte d’Azur, CNRS, OCA, Nice, France
²⁰ University of Lyon 1, ENS de Lyon, CNRS, Lyon, France
²¹ Research School of Astronomy and Astrophysics, Australian National University, Canberra 2611, Australia
²² Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
²³ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and U. de Chile, Casilla 36-D, Santiago, Chile
²⁴ The Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China

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Received: 26 May 2025
Accepted: 24 September 2025

Abstract

GRAVITY+ improves by orders of magnitude the sensitivity, sky-coverage, and contrast of the Very Large Telescope Interferometer (VLTI). A central part of this project is the development of Gravity Plus Adaptive Optics (GPAO), a dedicated high-order and laser-guide star adaptive optics (AO) system for VLTI. GPAO consists of four state-of-the-art AO systems that equip all 8 m class Unit Telescopes (UTs) for the wavefront correction of the VLTI instruments. It offers both visible and infrared natural guide star (NGS) and laser guide star (LGS) operations. The paper presents the design, operations, and performances of GPAO. We illustrate the improvement brought by GPAO with interferometric observations obtained during the commissioning of the NGS mode at the end of 2024. These science results include the first optical interferometry observations of a redshift z ~ 4 quasar, the spectroscopy of a cool brown-dwarf with magnitude K ~ 21.0, the first observations of a Class I young star with GRAVITY, and the first sub-micro arcsecond differential astrometry in the optical. Together with the entire GRAVITY+ project, the implementation of GPAO is a true paradigm shift for observing the optical Universe at very high angular resolution.