GRAVITY is an instrument on the

interferometer Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...

of the

Very Large Telescope The Very Large Telescope (VLT) is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with ...

(VLTI). It either combines the light of the four Unit Telescopes (UT) or the smaller four Auxiliary Telescopes. The instrument works with

adaptive optics Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in Astronomy, astronomical telescopes and laser communication systems to remove the effects of Astronomical seeing, atmo ...

and provides a resolution of 4

milliarcseconds A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

(mas) and can measure the position of astronomical objects down to a few 10 microarcseconds (μas). VLTI GRAVITY has a collecting area of 200 m² and the angular resolution of a 130 m telescope.

Instrument details

GRAVITY was built by a consortium led by the

Max Planck Institute for Extraterrestrial Physics The Max Planck Institute for Extraterrestrial Physics is part of the Max Planck Society, located in Garching, near Munich, Germany. In 1991 the Max Planck Institute for Physics and Astrophysics split up into the Max Planck Institute for Extraterr ...

. Other partner institutes are from

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

Portugal Portugal, officially the Portuguese Republic, is a country on the Iberian Peninsula in Southwestern Europe. Featuring Cabo da Roca, the westernmost point in continental Europe, Portugal borders Spain to its north and east, with which it share ...

and the

European Southern Observatory The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental organization, intergovernmental research organisation made up of 16 m ...

. The first light images included the discovery that Theta1 Orionis F in the

Trapezium Cluster The Trapezium or Orion Trapezium Cluster, also known by its Bayer designation of Theta1 Orionis (θ1 Orionis), is a tight open cluster of stars in the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo Galile ...

is a binary. GRAVITY can operate in single-field mode or in dual-field mode. In the dual-field mode it can interfere two astronomical objects at the same time and acquire this way very accurate

astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...

. The instrument data can also be used for K-band

spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...

with tree spectral resolutions. GRAVITY has the following sub-components: * IR wavefront sensing system CIAO (located at the Unit Telescopes) that will work with the MACAO deformable mirror * A polarisation control system to counteract polarisation effects in the VLTI * An active pupil guide system including

LED A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresp ...

sources mounted on each of the telescope

secondary mirror A secondary mirror (or secondary) is the second deflecting or focusing mirror element in a reflecting telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. Secondary mirro ...

support (spiders) * A field-guide system to track the position of the source * The Beam Combining Instrument (BCI) The Beam Combining Instrument is the primary unit of GRAVITY. It performs acquisition and provides interferometric fringes. BCI is

cryogenically In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a universa ...

cooled and located in the VLT-I laboratory.

Science

GRAVITY is mainly used to observe the stars orbiting the supermassive black hole

Sagittarius A* Sagittarius A*, abbreviated as Sgr A* ( ), is the supermassive black hole at the Galactic Center of the Milky Way. Viewed from Earth, it is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south o ...

and the position of

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

s and

brown dwarf Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...

s around their host star. It is also used for other studies that require a high resolution, such as the study of

circumstellar disks A Circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the rese ...

and the study of AGNs.

GRAVITY+

GRAVITY+ is the upgrade of GRAVITY, which will increase its sensitivity and increase its sky coverage. The upgrade is performed incrementally to reduce the disruption of astronomical observations.

References

{{reflist , refs= {{Cite web , title=ESO - Instrument Description , url=https://www.eso.org/sci/facilities/paranal/instruments/gravity/inst.html , access-date=2024-10-17 , website=www.eso.org {{Cite web , last=information@eso.org , title=GRAVITY Instrument Breaks New Ground in Exoplanet Imaging - Cutting-edge VLTI instrument reveals details of a storm-wracked exoplanet using optical interferometry , url=https://www.eso.org/public/news/eso1905/ , access-date=2024-10-17 , website=www.eso.org , language=en {{Cite journal , last1=Balmer , first1=William O. , last2=Pueyo , first2=Laurent , last3=Stolker , first3=Tomas , last4=Reggiani , first4=Henrique , last5=Maire , first5=A. -L. , last6=Lacour , first6=S. , last7=Mollière , first7=P. , last8=Nowak , first8=M. , last9=Sing , first9=D. , last10=Pourré , first10=N. , last11=Blunt , first11=S. , last12=Wang , first12=J. J. , last13=Rickman , first13=E. , last14=Kammerer , first14=J. , last15=Henning , first15=Th. , date=2023-10-01 , title=VLTI/GRAVITY Observations and Characterization of the Brown Dwarf Companion HD 72946 B , journal=The Astrophysical Journal , volume=956 , issue=2 , pages=99 , arxiv=2309.04403 , bibcode=2023ApJ...956...99B , doi=10.3847/1538-4357/acf761 , doi-access=free , issn=0004-637X {{Cite journal , last1=GRAVITY Collaboration , last2=Perraut , first2=K. , last3=Labadie , first3=L. , last4=Bouvier , first4=J. , last5=Ménard , first5=F. , last6=Klarmann , first6=L. , last7=Dougados , first7=C. , last8=Benisty , first8=M. , last9=Berger , first9=J. -P. , last10=Bouarour , first10=Y. -I. , last11=Brandner , first11=W. , last12=Caratti O Garatti , first12=A. , last13=Caselli , first13=P. , last14=de Zeeuw , first14=P. T. , last15=Garcia-Lopez , first15=R. , date=2021-11-01 , title=The GRAVITY young stellar object survey. VII. The inner dusty disks of T Tauri stars , url=https://ui.adsabs.harvard.edu/abs/2021A&A...655A..73G/abstract , journal=Astronomy and Astrophysics , volume=655 , pages=A73 , arxiv=2109.11826 , bibcode=2021A&A...655A..73G , doi=10.1051/0004-6361/202141624 , issn=0004-6361 {{Cite web , title=Science , url=https://www.mpe.mpg.de/372679/Science , access-date=2024-10-17 , website=www.mpe.mpg.de , language=en {{Cite web , title=First Light For Future Black Hole Probe , url=https://www.mpe.mpg.de/6499200/News_20160113 , access-date=2024-10-17 , website=www.mpe.mpg.de , language=en {{Cite journal , last1=Gravity+ Collaboration , last2=Abuter , first2=R. , last3=Alarcon , first3=P. , last4=Allouche , first4=F. , last5=Amorim , first5=A. , last6=Bailet , first6=C. , last7=Bedigan , first7=H. , last8=Berdeu , first8=A. , last9=Berger , first9=J. -P. , last10=Berio , first10=P. , last11=Bigioli , first11=A. , last12=Blaho , first12=R. , last13=Boebion , first13=O. , last14=Bolzer , first14=M. -L. , last15=Bonnet , first15=H. , date=2022-12-01 , title=The GRAVITY+ Project: Towards All-sky, Faint-Science, High-Contrast Near-Infrared Interferometry at the VLTI , url=https://ui.adsabs.harvard.edu/abs/2022Msngr.189...17A/abstract , journal=The Messenger , volume=189 , pages=17–22 , arxiv=2301.08071 , bibcode=2022Msngr.189...17A , doi=10.18727/0722-6691/5285 , issn=0722-6691 {{Cite web , last=information@eso.org , title=GRAVITY+ , url=https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/gravity+/ , access-date=2024-10-17 , website=www.eso.org , language=en {{Cite journal , last1=GRAVITY Collaboration , last2=Abuter , first2=R. , last3=Accardo , first3=M. , last4=Amorim , first4=A. , last5=Anugu , first5=N. , last6=Ávila , first6=G. , last7=Azouaoui , first7=N. , last8=Benisty , first8=M. , last9=Berger , first9=J. P. , last10=Blind , first10=N. , last11=Bonnet , first11=H. , last12=Bourget , first12=P. , last13=Brandner , first13=W. , last14=Brast , first14=R. , last15=Buron , first15=A. , date=2017-06-01 , title=First light for GRAVITY: Phase referencing optical interferometry for the Very Large Telescope Interferometer , url=https://ui.adsabs.harvard.edu/abs/2017A&A...602A..94G/abstract , journal=Astronomy and Astrophysics , volume=602 , pages=A94 , arxiv=1705.02345 , bibcode=2017A&A...602A..94G , doi=10.1051/0004-6361/201730838 , issn=0004-6361 {{Cite web , last=information@eso.org , title=GRAVITY , url=https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/gravity/ , access-date=2024-10-17 , website=www.eso.org , language=en Astronomical instruments Telescope instruments Spectrographs Interferometric telescopes Exoplanet search projects