The Giant Magellan Telescope (GMT) is a ground-based

extremely large telescope The Extremely Large Telescope (ELT) is an astronomical observatory currently under construction. When completed, it is planned to be the world's largest optical/near-infrared extremely large telescope. Part of the European Southern Observator ...

under construction, as part of the US Extremely Large Telescope Program (US-ELTP), . It will consist of seven 8.4 m (27.6 ft) diameter primary segments, that will observe optical and near infrared (320–25000 nm) light, with the resolving power of a 24.5 m (80.4 ft) primary mirror and collecting area equivalent to a 22.0 m (72.2 ft) one, which is about 368 square meters. The telescope is expected to have a resolving power 10 times that of the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

and four times that of the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...

, although it will be unable to image in the same infrared frequencies available to telescopes in space. , six mirrors have been cast and the construction of the summit facility has begun. A total of seven primary mirrors are planned, but it will begin operation with four. The

US$ The United States dollar (symbol: $; code: USD; also abbreviated US$ or U.S. Dollar, to distinguish it from other dollar-denominated currencies; referred to as the dollar, U.S. dollar, American dollar, or colloquially buck) is the official ...

1 billion project is US-led in partnership with Australia, Brazil, and South Korea, with Chile as the host country.

Site

The location of the telescope is

Las Campanas Observatory Las Campanas Observatory (LCO) is an astronomical observatory owned and operated by the Carnegie Institution for Science (CIS). It is in the southern Atacama Desert of Chile in the Atacama Region approximately northeast of the city of La Serena. ...

, which is also the site of the

Magellan Telescopes The Magellan Telescopes are a pair of optical telescopes located at Las Campanas Observatory in Chile. The two telescopes are named after the astronomer Walter Baade and the philanthropist Landon T. Clay. First light for the telescopes was on Se ...

, some north-northeast of

La Serena, Chile La Serena () is a city and commune in northern Chile, capital of the Coquimbo Region. Founded in 1544, it is the country's second oldest city after the national capital, Santiago. As of 2012, it had a communal population of roughly 200,000, and ...

and south of Copiapó, Chile, at an altitude of . The site has been chosen as the new instrument's location because of its outstanding

astronomical seeing In astronomy, seeing is the degradation of the image of an astronomical object due to turbulence in the atmosphere of Earth that may become visible as blurring, twinkling or variable distortion. The origin of this effect are rapidly changing v ...

and clear weather throughout most of the year. Moreover, due to the sparsity of population centers and other favorable geographical conditions, the night sky in most of the surrounding

Atacama Desert The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in the ...

region is not only free from

atmospheric pollution Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...

, but in addition it is probably one of the places least affected by

light pollution Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...

, making the area one of the best spots on Earth for long-term astronomical observation. Major site preparation began with the first blast to level the mountain peak on 23 March 2012. In November 2015, construction was started at the site, with a ground-breaking ceremony. The excavation for the foundations was completed in early 2019. As of August 2022, the site is ready for the pouring of the foundation's concrete.

Mirrors

The telescope will use seven of the world's largest mirrors as primary mirror segments, each in diameter. These segments will then be arranged with one mirror in the center and the other six arranged symmetrically around it. The challenge is that the outer six mirror segments will be off-axis, and although identical to each other, will not be individually radially symmetrical, necessitating a modification of the usual polishing and testing procedures. The mirrors are being constructed by the

University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a public land-grant research university in Tucson, Arizona. Founded in 1885 by the 13th Arizona Territorial Legislature, it was the first university in the Arizona Territory. T ...

Steward Observatory Steward Observatory is the research arm of the Department of Astronomy at the University of Arizona (UArizona). Its offices are located on the UArizona campus in Tucson, Arizona (US). Established in 1916, the first telescope and building were f ...

Richard F. Caris Mirror Lab. The casting of the first mirror, in a

rotating furnace A rotating furnace is a device for making solid objects which have concave surfaces that are segments of axial symmetry, axially symmetrical paraboloids. Usually, the objects are made of glass. The furnace makes use of the fact, which was known alr ...

, was completed on November 3, 2005, but the grinding and polishing were still going on 6 years later when the second mirror was cast, on 14 January 2012. A third segment was cast in August 2013, the fourth in September 2015, the fifth in 2017, and the sixth in 2021. The casting of each mirror uses 20 tons of E6

borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), ma ...

from the

Ohara Corporation is a Japanese global glass manufacturing company. The company is headquartered in Sagamihara with subsidiaries in a number of countries, including Japan, the United States, Germany, Hong Kong, Malaysia, Taiwan, and China, with Ohara Corporation ...

of Japan and takes about 12–13 weeks. After being cast, they need to cool for about six months. Polishing of the first mirror was completed in November 2012. As this was an off-axis segment, a wide array of new optical tests and laboratory infrastructure had to be developed to polish the mirror. The intention is to build seven identical off-axis mirrors, so that a spare is available to substitute for a segment being recoated, a 1–2 week (per segment) process required every 1–2 years. While the complete telescope will use seven mirrors, it is planned to begin operation with four mirrors. The primary mirror array as a whole will have a

focal ratio In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...

(focal length divided by diameter) of f/0.71. For an individual segment – being one third that diameter – this results in a focal ratio of f/2.14. The overall focal ratio of the complete telescope will be f/8 and the optical prescription is an aplanatic

Gregorian telescope The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke. James Gregory was a contemporary of Isaac Newton. Both often ...

. Like all modern large telescopes it will make use of

adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...

. Scientists expect very high quality images due to the very large aperture and advanced adaptive optics. Image

resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual mak ...

should exceed that of the

Support structure

The telescope structure is an alt-azimuth design and it will stand on a pier that is 22 meters in diameter. In late October 2019 GMTO announced the signing of a contract with German company MT Mechatronics (subsidiary of

OHB SE OHB SE is a European multinational technology corporation. Headquartered in Bremen, Germany, the corporation consists of the two business divisions Space Systems and Aerospace + Industrial Products. A key product of the corporation is fully int ...

) and Illinois-based

Ingersoll Machine Tools Ingersoll Machine Tools is a manufacturer located in Rockford, Illinois that produces large scale machine tools for use in metal cutting, 3D Printing, and automated fiber placement. History The company was founded in 1891 by Winthrop Ingersoll ...

, to design, build and install the GMT's telescope structure. The structure will weigh 1,800 tons without mirrors and instruments. With mirrors and instruments it will weigh 2,100 tons. This structure will float on a film of oil (50 microns thick), being supported by a number of hydrostatic bearings. The structure is expected to be delivered to Chile at the end of 2025. As of August 2022, construction of a 40,000 square foot facility at Ingersoll Machine Tools in Rockford, Illinois to build the telescope structure is complete, with the construction of the telescope structure in anticipation.

Wavefront control and adaptive optics (AO)

The primary mirrors are housed inside a "cell" which protect the mirrors.

Pneumatic actuator A pneumatic control valve actuator converts energy (typically in the form of compressed air) into mechanical motion. The motion can be rotary or linear, depending on the type of actuator. Principle of operation A Pneumatic actuator mainly co ...

s will push on the back of the primary mirrors to correct for the effects of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

variations on the mirrors. The GMT's

Adaptive Optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...

system will be built into the secondary mirrors which will be deformable. The Adaptive Secondary Mirrors (or ASMs) consist of a thin sheet of

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

that is bonded to more than 7000 independently controlled

voice coil A voice coil (consisting of a former, collar, and winding) is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it. The term ...

actuators. These

actuator An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". An actuator requires a control device (controlled by control signal) a ...

s will be able to push and pull on the mirrors over 1000 times a second to correct for wavefront distortions introduced by

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

in the Earth's atmosphere. The GMT will have several types of adaptive optics. The Ground layer AO allows corrections over a large field of view (≥ 10

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

). The Natural Guide Star AO is needed to produce diffraction-limited corrections over a small field of view (20-30

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

). The Laser Tomography AO uses six

laser guide star A laser guide star is an artificial star image created for use in astronomical adaptive optics systems, which are employed in large telescopes in order to correct atmospheric distortion of light (called '' astronomical seeing''). Adaptive op ...

s and a faint, natural

guide star In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a celestial body, whose apparent motion through the sky is primarily due to Earth's rotation. Accurate telescope pointing and tracking i ...

to extend diffraction-limited corrections to regions without a bright guide star. The performance will be similar to Natural Guide Star AO, but with reduced contrast.

Science instruments

The planned first light instruments are four instruments and one facility fiber positioning system. The fiber positioning system is necessary because of the wide field of view of the GMT. Using this system it is possible to observe multiple targets over the entire field with one or more of the spectrographs. * GMT-Consortium Large Earth Finder (G-CLEF) - an optical band echelle spectrograph * GMT Multi-object Astronomical and Cosmological Spectrograph (GMACS) - a visible multi-object spectrograph * GMT Integral-Field Spectrograph (GMTIFS) - a near-IR IFU and AO imager * GMT Near-IR spectrograph (GMTNIRS) - a near-IR spectrograph * The Many Instrument Fiber System (MANIFEST) - a facility fiber system Additionally the Commissioning Camera (ComCam) will be used to validate the Ground Layer

performance of the GMT facility Adaptive Optics System.

Comparison

The Giant Magellan Telescope is one of a new class of telescopes called

s with each design being much larger than previous telescopes. Other planned extremely large telescopes include the

Extremely Large Telescope The Extremely Large Telescope (ELT) is an astronomical observatory currently under construction. When completed, it is planned to be the world's largest optical/near-infrared extremely large telescope. Part of the European Southern Observator ...

and the

Thirty Meter Telescope The Thirty Meter Telescope (TMT) is a planned extremely large telescope (ELT) that has become controversial due to its location on Mauna Kea, on the island of Hawaiʻi. The TMT would become the largest visible-light telescope on Mauna Kea. S ...

Organizations

The project is US-led in partnership with Australia, Brazil, and South Korea, with Chile as the host country. The following organizations are members of the consortium developing the telescope. *

(Department of Astronomy and Steward Observatory) *

Arizona State University Arizona State University (Arizona State or ASU) is a public research university in the Phoenix metropolitan area. Founded in 1885 by the 13th Arizona Territorial Legislature, ASU is one of the largest public universities by enrollment in the ...

(School of Earth and Space Exploration (SESE)) *

Astronomy Australia Limited Astronomy Australia Limited (AAL) is an independent not-for-profit company whose members are all Australian universities and research organisations with a significant astronomy research capability. Since its incorporation in 2007, AAL has coordi ...

Australian National University The Australian National University (ANU) is a public research university located in Canberra, the capital of Australia. Its main campus in Acton encompasses seven teaching and research colleges, in addition to several national academies and ...

(Research School of Astronomy and Astrophysics) *

Carnegie Observatories The Carnegie Institution of Washington (the organization's legal name), known also for public purposes as the Carnegie Institution for Science (CIS), is an organization in the United States established to fund and perform scientific research. Th ...

University of São Paulo The University of São Paulo ( pt, Universidade de São Paulo, USP) is a public university in the Brazilian state of São Paulo. It is the largest Brazilian public university and the country's most prestigious educational institution, the best ...

(São Paulo Research Foundation – FAPESP) *

Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...

(jointly; Center for Astrophysics Harvard & Smithsonian (CfA)) * Korea Astronomy and Space Science Institute (한국천문연구원) (KASI) *

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded ...

(jointly; Center for Astrophysics Harvard & Smithsonian (CfA)) *

Texas A&M University Texas A&M University (Texas A&M, A&M, or TAMU) is a public, land-grant, research university in College Station, Texas. It was founded in 1876 and became the flagship institution of the Texas A&M University System in 1948. As of late 2021, T ...

University of Texas at Austin The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,075 ...

(Department of Astronomy at the University of Texas at Austin) *

University of Chicago The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the b ...

(Astronomy & Astrophysics Department of the University of Chicago) The Carnegie Observatories office in Pasadena has an outline of the GMT primary mirror array painted in its parking lot. It is easily visible in satellite imagery at . In January 2018, WSP was awarded the contract to manage construction of the GMT.

Status of mirrors

There will be a total of eight primary mirror segments: one central mirror, six off-axis segments, and a spare off-axis segment which will be rotated into use as each segment is cleaned and recoated. The mirrors are made of

and have a

honeycomb structure Honeycomb structures are natural or man-made Structure, structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight and minimal Material costs, material cost. The geometry of ...

below the mirror surface. An adaptive secondary mirror is also designed for the telescope. The telescope will begin observing with only four mirrors: the central and three off-axis segments. * Mirror 1, cast in October 2005, completed in August 2012 with polishing completed with a surface accuracy of 19 nanometers RMS. * Mirror 2, cast in January 2012, completed in 2019. * Mirror 3, cast in August 2013, currently (August 2022) receiving the final touches on its polishing and is in final testing. * Mirror 4, cast in September 2015. This is the central mirror. Currently (March 2021) the rear surface is polished and is having its load spreaders installed. * Mirror 5, cast in November 2017. * Mirror 6, cast in March 2021. Approximated to take 4 years to complete. * Mirror 7, in planning, will be cast in 2023. * Mirror 8, planned. The secondary mirror, which is a deformable mirror tasked with correcting the atmospheric distortion of the light gathered by the telescope, has 7 segments and 1.1 meter diameter. The first segment is under construction as of August 2022.

References

External links

Giant Magellan Telescope home page

Lecture by director Patrick McCarthy on technologies behind GMT

''New Scientist'' article on the telescope

COSMOS article on the telescope

Painted outline of the mirrors at the parking lot of the Observatories of the Carnegie Institution
in

Pasadena Pasadena ( ) is a city in Los Angeles County, California, northeast of downtown Los Angeles. It is the most populous city and the primary cultural center of the San Gabriel Valley. Old Pasadena is the city's original commercial district. Its ...

J. Rosenberg - Seeing Stars (2013) - ''Harvard Magazine''

{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Optical telescopes Telescopes under construction Astronomical observatories in Chile