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The Messenger ELT M4 — The Largest Adaptive Mirror Ever Built The Messenger A Celebration of GRAVITY Science The ESO Summer Research Programme 2019 No. 178 –Quarter 4| 178 No. 2019 ESO, the European Southern Observa- Contents tory, is the foremost intergovernmental astronomy organisation in Europe. It is Telescopes and Instrumentation supported by 16 Member States: Austria, Vernet E. et al. – ELT M4 — The Largest Adaptive Mirror Ever Built 3 Belgium, the Czech Republic, Denmark, Kasper M. et al. – NEAR: First Results from the Search for Low-Mass ­ France, Finland, Germany, Ireland, Italy, Planets in a Cen 5 the Netherlands, Poland, Portugal, Spain, Arnaboldi M. et al. – Report on Status of ESO Public Surveys and Sweden, Switzerland and the United Current Activities 10 Kingdom, along with the host country of Ivanov, V. D. et al. – MUSE Spectral Library 17 Chile and with Australia as a Strategic Partner. ESO’s programme is focussed GRAVITY Science on the design, construction and opera- GRAVITY Collaboration – Spatially Resolving the Quasar Broad Emission tion of powerful ground-based observing Line Region 20 facilities. ESO operates three observato- GRAVITY Collaboration – An Image of the Dust Sublimation Region in the ries in Chile: at La Silla, at Paranal, site of Nucleus of NGC 1068 24 the Very Large Telescope, and at Llano GRAVITY Collaboration – GRAVITY and the Galactic Centre 26 de Chajnantor. ESO is the European GRAVITY Collaboration – Spatially Resolved Accretion-Ejection in partner in the Atacama Large Millimeter/ Compact Binaries with GRAVITY 29 submillimeter Array (ALMA). Currently GRAVITY Collaboration – Images at the Highest Angular Resolution ESO is engaged in the construction of the with GRAVITY: The Case of h Carinae 31 Extremely Large Telescope. Wittkowski M. et al. – Precision Monitoring of Cool Evolved Stars: Constraining Effects of Convection and Pulsation 34 The Messenger is published, in hardcopy GRAVITY Collaboration – Multiple Star Systems in the Orion Nebula 36 and electronic form, four times a year. GRAVITY Collaboration – Probing the Discs of Herbig Ae/Be Stars at ESO produces and distributes a wide Terrestrial Orbits 38 variety of media connected to its activi- GRAVITY Collaboration – Spatially Resolving the Inner Gaseous Disc of the ties. For further information, including Herbig Star 51 Oph through its CO Ro-vibration Emission 40 postal subscription to The Messenger, Davies C. L. et al. – Spatially Resolving the Innermost Regions of the contact the ESO Department of Commu- Accretion Discs of Young, Low-Mass Stars with GRAVITY 43 nication at: Dong S. et al. – When the Stars Align — the First Resolved Microlensed Images 45 GRAVITY Collaboration – Hunting Exoplanets with Single-Mode ESO Headquarters Optical Interferometry 47 Karl-Schwarzschild-Straße 2 85748 Garching bei München, Germany Astronomical News Phone +498932006-0 Christensen L. L., Horálek P. – Light Phenomena Over ESO’s Observatories IV: [email protected] Dusk and Dawn 51 Manara C. F. et al. – The ESO Summer Research Programme 2019 57 The Messenger Boffin H. M. J. et al. – Report on the ESO Workshop Editor: Gaitee A. J. Hussain “Artificial Intelligence in Astronomy” 61 Layout, Typesetting, Graphics: Vieser W. et al. – Report on the IAU Conference Jutta Boxheimer, Mafalda Martins “Astronomy Education — Bridging Research & Practice” 63 Design, Production: Jutta Boxheimer Kokotanekova R., Facchini S., Hartke J. – Fellows at ESO 67 Proofreading: Peter Grimley In Memoriam Cristian Herrera González 70 www.eso.org/messenger/ Personnel Movements 71 Patat F. – Erratum: The Distributed Peer Review Experiment 71 Printed by FIBO Druck- und Verlags GmbH Fichtenstraße 8, 82061 Neuried, Germany Unless otherwise indicated, all images in The Messenger are courtesy of ESO, except authored contributions which are courtesy of the respective authors. Front cover: Simulation of the orbits of stars very close to the supermassive black hole at the heart of © ESO 2019 the Milky Way, Sgr A*. One of these stars, S2, is the ISSN 0722-6691 perfect laboratory to test Einstein’s general theory of relativity as it passes very close to the black hole, with an orbital period of 16 years. S2’s orbit has been monitored with ESO’s telescopes since the 1990’s and continues at even greater precision with GRAVITY. Credit: ESO/L. Calçada/spaceengine.org 2 The Messenger 178 – Quarter 4 | 2019 Telescopes and Instrumentation DOI: 10.18727/0722-6691/5162 ELT M4 — The Largest Adaptive Mirror Ever Built Elise Vernet 1 (approximately a ninth of the full moon). Michele Cirasuolo 1 Thanks to the combined use of M4 and Marc Cayrel 1 M5, the optical system is capable of Roberto Tamai 1 correcting for atmospheric turbulence AdOptica/ESO Aglae Kellerer 1 and the vibration of the telescope struc- Lorenzo Pettazzi 1 ture itself induced by motion and wind. Paul Lilley 1 Pablo Zuluaga 1 This adaptive capability is crucial to Carlos Diaz Cano 1 allowing the ELT to reach its diffraction Bertrand Koehler 1 limit, which is ~ 8 milliarcseconds (mas) in Fabio Biancat Marchet 1 the J-band (at λ ~ 1.2 μm) and ~ 14 mas Juan Carlos Gonzalez 1 in the K-band. In so doing the ELT will Mauro Tuti 1 be able to yield images 15 times sharper + the ELT Team than the Hubble Space Telescope and with much greater sensitivity. Translated into astrophysical terms this means 1 ESO opening up new discovery spaces, from Figure 1. Rendering of the M4 adaptive mirror unit exoplanets closer to their stars, to black for the ELT. holes, to the building blocks of galaxies The Extremely Large Telescope (ELT) is both in the local Universe and billions of consortium name of AdOptica. Many at the core of ESO’s vision to deliver the light years away. For example, the ELT 8-metre telescopes now have a metre- largest optical and infrared telescope will be able to detect and characterise scale adaptive mirror. The same tech- in the world. Continuing our series of extrasolar planets in the habitable zone nology is now being adapted to serve the Messenger articles describing the opti- around our closest star Proxima Centauri, ELT, in order to produce a mirror with an cal elements of the ELT, we focus here or to resolve giant molecular clouds (the area five times larger. The M4 mirror uses on the quaternary mirror (M4), a true building blocks of star formation) down to the same principle as a loudspeaker; the technological wonder; it is the largest ~ 50 parsecs in distant galaxies at z ~ 2 mirror is made of a very thin shell levitating deformable mirror ever made. In combi- (and even smaller structures for sources 100 microns away from its reference sur- nation with M5, M4 is vital to delivering that are gravitationally lensed by fore- face (this corresponds to the thickness the sharp (diffraction-limited) images ground clusters) with an unprecedented of a standard A4 sheet of paper) and it needed for science by correcting for sensitivity. acts like a membrane which deforms atmospheric turbulence and the vibra- under the effect of about 5000 voice coil tions of the telescope itself. Here we actuators. A voice coil actuator is a type describe the main characteristics of M4, The quaternary mirror (M4) of direct drive linear motor and the name the challenges and complexity involved “voice coil” comes from one of its first in the production of this unique adaptive M4 is the main adaptive mirror of the tele- historical applications, vibrating the paper mirror, and its manufacturing status. scope. The term “adaptive mirror” means cone of a loudspeaker. It consists of a that its surface can be deformed to cor- permanent magnetic field assembly and rect for atmospheric turbulence, as well a coil assembly. The current flowing Background: how the ELT works as for the fast vibration of the telescope through the coil assembly interacts with structure induced by its motion and the the permanent magnetic field and gener- Let’s briefly recall how the ELT works. wind. In the case of M4, more than 5000 ates a force that can be reversed by The optical design of the ELT is based on actuators are used to change the shape changing the polarity of the current. a novel five-mirror scheme capable of of the mirror up to 1000 times per second. collecting and focusing the light from Depending on the current injected into astronomical sources and feeding state- In combination with the M5 mirror, M4 the coil the mirror can be pushed or of-the-art instruments for the purposes of forms the core of the adaptive optics of pulled up to a distance of 90 microns imaging and spectroscopy. The light is the ELT. With a diameter of 2.4 metres, from its mean position. With the help of collected by the giant primary mirror M4 will be the largest adaptive mirror ever a very fast and precise set of capacitive 39 metres in diameter, relayed via the M2 built. By comparison, current adaptive sensors and amplifiers that are co-located and M3 mirrors (each of which has a mirrors are just over 1 metre in diameter, with the voice coil actuators, the mirror’s diameter of ~ 4 metres) to the M4 and M5 for example the 1.1-m M2 adaptive sec- position is measured 70 000 times per mirrors that form the core of the adaptive ondary on the VLT UT4 telescope (Yepun). second to an accuracy of a few tens of optics of the telescope; the light then nanometres (the size of the smallest virus) reaches the instruments on one or other Adaptive mirror technology was trans- with the actuators being driven up to of the two Nasmyth platforms. This lated into an industrial product for astron- 1000 times per second. design provides an unvignetted field of omy more than two decades ago by view (FoV) of 10 arcminutes in diameter the Italian companies Microgate s.r.l and M4 is made of several state-of-the-art on the sky, ~ 80 square arcminutes ADS, internationally known under the components, the mirror and its reference The Messenger 178 – Quarter 4 | 2019 3 Telescopes and Instrumentation Vernet E.
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