Frequently Asked Questions
What is the GMT?
Q: What is the Giant Magellan Telescope?
A: The Giant Magellan Telescope (GMT) will be the first in a new class of Extremely Large Telescopes, capable of exploring the cosmos with unprecedented clarity and sensitivity. The GMT will peer back in time to shortly after the Big Bang, when the first stars, galaxies, and black holes formed. The GMT will leverage cutting-edge optics technology to combine seven giant mirrors to achieve 10 times the angular resolution of the Hubble Space Telescope in the infrared region of the spectrum. When it begins operations in the next decade, GMT will embark on a mission of discovery to explore the origins of the chemical elements (carbon, oxygen, nitrogen and others) that make up our planet and our bodies, the formation of the first stars to form in the universe, and the mysteries of dark matter and dark energy. The GMT will also search distant exoplanets for signs of biological processes around other stars in our Milky Way galaxy.
Q: What is the current status of the project?
A: After a comprehensive review of systems-level designs by an external panel, the GMT Project formally entered its construction phase in 2015. Prior to this announcement, the preparation of the GMT mountaintop site in the Chilean Andes began in 2012, and the Richard F. Caris Mirror Lab began casting primary mirror segments in 2005. Work is underway, in one stage or another, on each of the seven GMT mirrors.
Q: What are the breakthrough technologies developed for GMT?
A: The primary mirrors are the breakthrough technology for the GMT. They are the most challenging optics ever produced (in technical terms they are large, fast, off-axis aspheres). When coupled with the GMT adaptive optics (AO) system they will produce images sharper than those from the Hubble or Webb Space Telescopes.
Q: Which adaptive optics (AO) modes will be supported?
A: We will provide diffraction-limited observing modes in the first few years of operations. The primary mode of AO for the GMT is expected to be Laser Tomography Adaptive Optics (LTAO). Six sodium laser beacons will be used in conjunction with natural stars to produce diffraction-limited images in the near- and mid-infrared over fields 30-90 arcseconds in diameter. The adaptive secondary mirror will act as the deformable mirror in the AO system. Additional deformable mirrors can be added to allow Multi-Conjugate Adaptive Optics (MCAO), increasing the field of view and improving the uniformity of the images. During the design phase, the project will explore costs and implementation strategies for this mode. The other principal AO modes are expected to be Ground-Layer Adaptive Optics (GLAO) and Extreme Adaptive Optics (ExAO). GLAO will provide a 50-200% reduction in image size in the near-infrared over large fields of view. ExAO provides extremely high contrast over small fields of view and is important to a number of key GMT science goals. We expect that ExAO will be developed for the second generation of GMT instruments. It is likely that high-contrast imaging will happen first in the mid-infrared, where there are both scientific and technical advantages, followed by an evolution to shorter wavelengths. On longer time scales the project will continue to examine other innovative AO modes, such as Multi-Object Adaptive Optics (MOAO) and other forms of deployable AO.
Q: What are the advantages of the segmented adaptive secondary?
A: The use of adaptive secondary mirrors allows us to implement adaptive optics without additional background or throughput losses. The Gregorian optical prescription allows us to correct for atmospheric turbulence over a wide field of view and to utilize a variety of correction modes including diffraction-limited and ground layer correction. The segmentation of the adaptive secondary mirrors allows us to transfer large scale motions from the primary mirror segments to the much more agile secondary mirrors. Rather than moving 17 tons of glass to compensate for phase errors in each segment we only need to move roughly 2kg of glass at the secondary mirrors. This greatly simplifies the overall optical control challenge and provides more pathways towards realizing the exquisite imaging potential of the telescope.
Q: How will the telescope be phased?
A: The telescope will be phased (i.e. brought into optical alignment) by positioning and pistoning the primary and secondary mirror segments. Stars will be observed continuously in the near-infrared, where the atmospheric coherence length is larger than the gap between mirror segments. Interference fringes are formed at the mirror gaps and these are used to sense average phase differences on time scales of about one minute. Optical or mechanical edge sensors that span the gaps between segments of both the primary and secondary mirrors will be used to track deviations from the average phase caused by mechanical vibrations on time scales shorter than one minute. Adjustments on shorter time scales will be made with the adaptive secondary mirror segments, while drifts on longer time scales are taken out with the primary mirror segment supports. Sky coverage is nearly 100% using currently available near-infrared and optical wavefront sensors.
Q: What are the first generation instruments?
A: We are developing four instruments and one facility fiber optics positioning system. These range from very powerful multi-object spectrographs operating in the visible and the near-infrared, to high-resolution spectrometers and AO-fed imagers and spectrographs working in the near- and mid-infrared. These instruments are described in the GMT website at http://www.gmto.org/resources/#science-instruments. At first light, we expect to have two instruments ready to be commissioned with the telescope. Once the telescope is commissioned, two more instruments plus the fiber optics positioner will be added. New instruments will be developed all the time; we would expect a new instrument every few years once the telescope is operational.
Q: What is the light-collecting ability of the GMT compared to other telescopes?
A: When it is fully commissioned, the GMT will have ten times the resolution of the Hubble Space Telescope (4 times the resolution of the James Webb Space Telescope) and 7-10 times the light-gathering power of the largest telescopes today.
Chile and Las Campanas
Q: Where is the GMT being built?
A: The telescope will be built at Las Campanas Observatory (LCO) in the Andes mountain range in the Atacama Desert in Chile. At an elevation of 2,500 meters (8,200 feet) and latitude 29° South, Las Campanas is known for its dark skies, excellent weather, and outstanding seeing. Extensive site testing, both in advance of the construction of the twin Magellan 6.5 meter telescopes and for the GMT, has shown that the images are quite sharp (half a second of arc). While Las Campanas is a remote site far from any towns or cities, it has a well-developed infrastructure of roads, electrical power, and water from deep wells fed by runoff from the high Andes mountain range.
Q: What are the benefits for Chile for hosting the GMT?
A: The GMT will be located on the top of Las Campanas, the mountain that gives the name to the Las Campanas Observatory. In 1971 the first of the telescopes owned and operated by Carnegie Institution for Science was opened to Carnegie astronomers and the Chilean community. Since then a number of smaller telescopes have operated at Las Campanas, making it one of the premier observatories on the planet. By the agreement with the University of Chile, all telescopes on Las Campanas, including the GMT, are available to professional astronomers working at Chilean institutions. A comprehensive article discussing the benefits of astronomy to the economy of Chile is available from the AAAS Center for Science Diplomacy.
Q: Are you working with the Chilean government?
A: The GMTO organization has a cooperative agreement with the University of Chile and is recognized by the Government of Chile as a special international organization. The GMT team and our partner universities and institutions have active outreach activities in Chile and work closely with the Chilean Universities and the relevant departments of the Government of Chile.
Q: How many jobs will you be creating in Chile with this program?
A: During construction, we expect to have up to 250 workers on the site building the telescope and related infrastructure. The GMTO office in Santiago will have 20-40 staff during construction. During the operations phase (expected to be at least 50 years) we expect to employ 100-150 Chileans in technical, administrative and support positions.
Q: Are there any natural hazards at the Las Campanas site?
A: All of the world’s great observatories are perched atop high mountains. These mountains are made either from volcanic or plate tectonic processes and thus are inherently risky. We have designed the GMT to withstand the largest earthquakes expected in a 2,500-year period at the Las Campanas site. We expect the telescope to be operating for a very long time!
Q: Do you have permits to build on Las Campanas?
A: One of our founding partners, the Carnegie Institution for Science, has owned the Las Campanas site since the late 1960s. We have an excellent relationship with the government of Chile and local authorities and have all of the necessary permissions in place.
Q: Where are the primary mirrors being made?
A: The mirrors are being produced by the Richard F. Caris Mirror Lab of the Steward Observatory of the University of Arizona, one of the partner institutions of GMTO. The telescope’s 24.5 meter (82 foot) primary mirror will be comprised of seven separate 8.4 meter (27 foot) diameter segments. Each of the primary mirror segments weighs approximately 17 tons and takes a year to cast and cool. After casting, the fabrication of each segment requires more than three years of surface generation and meticulous polishing.
Q: How are the mirrors made and what are they made from?
A: This article from Gizmodo has a great Q&A with Mirror Lab scientist Buddy Martin on how the GMT mirrors are made. In addition, an updated article is available on The Conversation, along with a great YouTube video describing the mirror casting process.
The glass that we use is made by the Ohara Corporation of Japan. It is a borosilicate glass with a very low, and very uniform, rate of thermal expansion. It takes 20 tons of glass to make one of our mirrors, so we will need 160 tons in the end to make the seven mirrors and the operating spare. The mirrors are coated with a film of Aluminum that is only a few atoms thick. It is one of the strange aspects of telescope building that we build hundreds of tons of steel and glass to hold a few grams of Aluminum in just the right shape so we can collect the few photons that have traveled billions of years to reach our telescope!
Q: How long does it take to build a mirror?
A: It takes more than four years, from start to finish. The Mirror Lab, which is under the football stadium at the University of Arizona, can work on three mirrors at a time. The first stage is the mirror blank – this takes a matter of hours to melt the glass, but then a few months for it to anneal (the slow process of cooling the mirror and relieving internal stress). Next, the mirror is taken out of the furnace and tilted on its side so all the mold materials can be cleaned out. This process takes another few months. Then, the mirror is turned on its front and the back is polished on the “LOG” – Large Optical Generator. This takes about 6 months. Then the mirror is turned on its back and the hard work begins. Hardware is attached to the back of the mirror to support it during polishing and when in the telescope. The front of the mirror is then “roughly” ground for about 6 months, then the fine polishing begins – this latter step can take about a year and is done very carefully. The mirror is measured many times during this process – this involves taking the mirror off the polishing machine and moving it under the test tower. We follow the old adage of “measure twice, grind once!”
Watch a timelapse of the glass melting inside the Segment 5 furnace in 2017 in this video.
Q: What’s the advantage of spin casting the mirrors? Does spin-casting still work for off-axis mirrors?
A: Spin casting saves a lot of grinding of excess glass from the mirror surface as the mirror comes out of the mold in a parabolic shape. It also reduces the annealing time – the slow cooling of the glass to remove stress. Even though 6 of GMT’s mirrors are off-axis it still helps a lot! When the off-axis mirrors come out of the furnace the top plate is not of uniform thickness due to the off-axis shape and the fact that the center of the furnace rotation is in the center of the mirror blank. By the time we are done polishing the mirror the top plate has a uniform thickness and the mirror matches the desired off-axis shape.
Q: What are the advantages of the large primary mirror segments?
A: The GMT segments will provide the largest possible area of continuous wavefront and will reject wind-driven disturbances. The manufacturing, mechanical support, and thermal control of these mirrors are well understood and this knowledge is contained within the GMT partner institutions. The project has invested in the infrastructure at the Mirror Lab to produce these segments in an efficient manner. The first mirror segment was completed in 2012 and demonstrated the viability of polishing large off-axis mirrors to the required optical tolerances. The GMT primary segments have a heritage in the MMT, Magellan and Large Binocular Telescope primary mirrors. These mirrors have been shown to produce excellent images. They have high mechanical stiffness and short thermal time scales.
Q: How is the polishing process going?
A: Five of the seven primary mirror segments have been cast at the Richard F. Caris Mirror Lab at the University of Arizona. This is the status of the mirror production as of April 2019:
- Segment 1 was completed in 2012 and was moved into temporary storage near Tucson Airport in September 2017. The mirror is named for George P. and Cynthia Woods Mitchell.
- Segment 2 was completed in 2019 and was moved into temporary storage near Tucson Airport in July 2019. The mirror is named for George P. and Cynthia Woods Mitchell.
- Segment 3 is undergoing fine grinding of its front surface.
- Segment 4 has had its rear surface polished and having its load spreaders installed.
- Segment 5 was cast on November 4, 2017 and is now undergoing rear surface grinding.
- Segment 6 was cast on March 5, 2021.
- Segment 7 is planned to be cast in 2023.
The steps to creating a GMT mirror are outlined in this infographic.
Q: How do you get the mirrors to Chile and up the mountain?
A: We will most likely transport the mirrors from Tucson, Arizona, by road to a US port. Then they will be transported by ship to Chile and finally by road to the site at Las Campanas.
Q: Why did you decide to formally move forward with construction in 2015?
A: The idea behind the Giant Magellan Telescope has been maturing for some time. In 2014 the project completed a rigorous set of reviews that evaluated the project’s design plan and budget. Having passed the inspection with flying colors, and having raised more than $500 million dollars (US) for construction, the Giant Magellan Telescope Board of Directors unanimously voted to move into the construction phase of this historic endeavor.
Q: What pre-construction work has taken place?
A: We realized early in the project that we needed to convince ourselves that we could make the challenging optics at the heart of the GMT. In late 2012 we completed the first of the GMT mirrors and thus retired our greatest technical risk. In addition, we began preparing the mountaintop site in the Chilean Andes for construction in mid-2012. This preconstruction work also allowed us to measure the properties of the rock under the soil and so finalize our designs for the foundation of the observatory. A telescope that looks up at the heavens must be firmly anchored to the Earth. Other pre-construction work has already taken place, including grading the roads, constructing the workers’ residence and dining facilities, and a summit office.
Q: What will be included in the first phase of construction?
A: The first phase of construction will see the completion of the telescope – a precision optical and mechanical instrument that collects light from celestial objects and delivers it to a focus – and the ‘dome’ – a rotating cylindrical building that protects the telescope during the day. In addition, we will build an initial suite of scientific instruments including sophisticated cameras and spectrographs that will allow scientists to measure the composition and dynamics of distant planets and galaxies. A support campus for the technical staff and visiting astronomers will also be constructed.
Q: What will not be included in the first phase of construction?
A: Some of the more sophisticated optical instruments and advanced optical correction technology are not slated for construction during the initial phase. We will continue to develop these technologies, however, and when they are mature we will include them in the construction plan. These advanced instruments and correcting optics will further enhance the power of the observatory.
Q: What major construction contracts will be needed to complete the telescope?
A: Essentially all of the large components will be developed through procurement contracts. The largest contracts are for the optics, the telescope steel structure, and the rotating enclosure. The primary mirror optics are already under contract and we signed contracts for the development of the telescope mount in 2017. The enclosure construction is likely to be done via a number of contracts.
Q: What is the oversight and review process for the project?
A: The project has gone through the customary large scale review process with a conceptual design review and a preliminary design review before the start of construction. Detailed subsystem reviews are held regularly and a standing review board comprised of internationally recognized experts in the development of telescopes, optics and complex scientific facilities reviews technical and programmatic status on a regular basis. Key stakeholders are apprised of the review recommendations and management’s response to these. We strive for openness, transparency, and input from a diverse set of experts.
History and Timing
Q: What is the history of the GMT? Who made the decision to start the project?
A: The GMT concept, as with those of the other extremely large telescope projects, has its roots in turn-of-the-millennium discussions about how to build on the successes of the 8-10 m telescopes developed in the 1980s and 1990s. Three quite distinct primary mirror technologies – segmented mirrors (Keck), monolithic meniscus mirrors (Gemini, VLT, Subaru), and structured monolithic mirrors (MMT, Magellan) – had, by 2000 or shortly thereafter, produced telescopes that yielded outstanding image quality at costs well below the extrapolation of the D2.7 scaling (D is the diameter of the primary mirror) from the 1950-1970’s 2 m to 4 m aperture telescopes. The Magellan telescopes (Baade in 1999 and Clay in 2001) produced exceptionally good images at a site that, while known for good seeing, was not until then considered a rival in terms of image quality to Mauna Kea in Hawaii.
The excellent images from the Magellan telescopes arise from a combination of well-figured primary mirrors, precise and accurate thermal control, and a full-time active optics system that corrects many aberrations in the primary mirror and repositions the primary and secondary mirrors on time scales of about a minute. The short thermal time scale (about 20 minutes) of the Arizona mirrors allows them to be controlled at a level that tracks the ambient air temperature with high accuracy.
It was clear that apertures larger than 8-9 m could only be achieved by segmenting the primary mirror collecting area. Basic material properties make the fabrication of monoliths larger than 9 m difficult, and the challenges of transporting and handling such large optics quickly become insurmountable. It was also clear that the cost scaling with diameter must be beaten to reach 20-30 m apertures at affordable costs. Scaling the cost of a 25 m aperture from the 4 m costs using either the D2.7 scaling or the more recent D2.4 scaling leads to very high-cost estimates. Breaking the cost scaling would likely require primary focal ratios even faster than the very fast – f/1.25 (MMT, Magellan) to f/2 (VLT) – primaries on the 8-10 m telescopes.
The GMT concept arose from the ’20/20′ concept for an interferometer using twin 21 m telescopes on a circular track 100 m in diameter. The individual telescopes were composed of seven 8.4 m diameter segments with a primary focal ratio of f/0.7. The concept for the individual telescopes in the 20/20 interferometer formed the starting point for the GMT design.
The GMT is being developed by an international consortium of universities and research institutions, starting in 2004 with the Carnegie Institution, the University of Arizona, Harvard University, and the Smithsonian Institution. The University of Texas at Austin and Texas A&M University joined the project in 2006. They were followed by the Korea Astronomy and Space Science Institute (KASI), the Australian National University, and Astronomy Australia Limited in 2008. That same year the GMT founding institutions signed a partnership agreement and formed an independent nonprofit corporation to manage the development, construction, and operation of the GMT. The University of Chicago joined the project in 2010 and FAPESP, representing the state of São Paulo, Brazil, joined in 2014. Arizona State University joined the project in 2017.
Q: Why build the Giant Magellan Telescope now?
A: The time is right to build the next-generation astronomical observatory. We have a clear technical path forward and a burning set of scientific questions that can only be addressed with a giant telescope. These questions are couched in sophisticated scientific language, but can be reduced to questions that we can all identify with: What is the Universe made of? Where did we come from? Are we alone in the Cosmos? Answering these questions involves studying the properties of planets around other stars, the early formation of galaxies, stars, and planetary systems, the nature of dark energy and dark matter, and the evolution of the elements.
Q: When will construction be complete?
A: The project is structured around a staged implementation plan. We are projecting that we will have the telescope with a subset (3-4) segments in place in late 2026. The remaining segments will reach the mountain at the rate of about 1 a year.
Q: What is the lifetime of the Observatory?
A: There is nothing that will naturally limit the lifetime of the observatory. We are designing for a minimum 50-year lifetime. Some past observatories have lost their impact due to man-made light pollution from nearby cities, but the GMT is being built at a site with little likelihood of future light pollution.
Q: What astronomy will the GMT specialize in?
A: The GMT will cover a very broad range of astrophysics, but it will specialize in several areas of astronomy such as the formation of stars and planetary systems, the properties of exoplanetary systems, stellar populations and chemical evolution, galaxy assembly and evolution, dark matter, dark energy and fundamental physics, first light and reionization, and transient phenomena. For more information about the GMT Science Case, please check out our 2018 Science Book (pdf).
Q: Will the GMT be able to take photographs of Earth-like planets around other stars?
A: Once the near-infrared integral field unit spectrograph and adaptive optics system are in place, the GMT will be able to image exoplanets. Taking photographs of planets like the Earth will be technically challenging and will likely require further innovations in optics. Scientists in our community are actively working on this challenge.
Q: Why is the GMT an important milestone for the scientific community?
A: As we start construction of the GMT, the largest telescope in history when it begins operations, we are taking a huge step forward for humanity’s understanding of the universe. We are ramping up a project that will give us the ability to see deeper into space than ever before, opening the door to discovery.
Q: What are the plans for data archives and support for archival research?
A: The baseline GMT operations plan calls for archiving data, and requires that all instruments provide a means of ensuring that all relevant metadata are attached and that all GMT data are cataloged in a format making them useful to scientists around the world.
Q: As an astronomer, can I expect to have access to the GMT?
A: The GMT project is seeking additional partners, and is open to institutions from around the world. Partners can join during the construction phase (‘Founders’) or can help to support operations (‘Participants’). The model for sharing observing time means that the earlier in the project a new partner joins, the better value-for-money their investment returns. The GMT Board has been in discussions with the US National Science Foundation to ensure open US access to the GMT in proportion to US public investment. This investment could be in the form of contributions to the capital construction costs, to annual operations or to instrumentation development. AURA is acting as the interface between the US community, the NSF and the US extremely large telescope projects. You can learn more by visiting the US Extremely Large Telescope Program website at https://nationalastro.org/USELTP/.
In Australia, public participation is being coordinated by Astronomy Australia Limited and you can learn more by visiting http://astronomyaustralia.org.au. The Korean Astronomy and Space Science Institute (KASI; https://www.kasi.re.kr/eng/index) is currently acting as the responsible agency in Korea. If you would like to ensure access to the GMT for you and your colleagues be sure to let your national agency know and become involved in community activities relating to extremely large telescopes.
Q: As an institution, how can we get involved in instrumentation development?
A: The majority of the instrumentation development takes place at our Founder institutions and their collaborators. Find our list of Founders here: http://www.gmto.org/partners/.
Extremely Large Telescope Questions
Q: How many other Extremely Large Telescope projects exist?
A: Currently there are three ELT projects: the Giant Magellan Telescope (GMT) project (http://www.gmto.org), the Thirty Meter Telescope (TMT) project (http://www.tmt.org), and the European Extremely Large Telescope (ELT) project (www.eso.org/e-elt).
Q: Are the three Extremely Large Telescope projects competing with one another?
A: While the three ELTs have different features and strengths, all three ELTs represent dramatic advances over currently existing telescopes. From a design perspective, the GMT will have seven very large primary mirrors while the TMT and the European ELT will comprise hundreds of smaller mirrors. All three ELTs will likely strive to answer the same fundamental scientific questions, which opens the door for collaboration as well as competition. While we are technically competing to be first on the sky, we hope that all three telescopes will be built and be operational in the next decade.
Q: Why does GMT want to be the first Extremely Large Telescope?
A: Astronomy is about exploration. Being the first to enter uncharted territory provides one with unique opportunities for discovery. There are a number of prime scientific questions just waiting for a giant telescope like the GMT.
Q: Will the GMT work with the other ELTs and future ground-based telescopes?
A: As with all modern astronomy, it often takes many telescopes to fully exploit a discovery. It is likely that astronomers will use the GMT and other ELTs to perform detailed follow-up work on observations made by other telescopes. For example, the LSST will be starting its 10-year sky survey when the GMT has first light, and we expect the GMT to be in a position to work with the LSST to make discoveries. The different strengths of the ELTs mean that there are opportunities to work together efficiently.
Q: Will the Giant Magellan Telescope work with the James Webb Space Telescope?
A: Space telescopes and ground-based telescopes have always complemented each other. For example, while the Hubble Space Telescope takes many spectacular images, it is the detailed follow-up work by the 8m-class ground-based telescopes such as Gemini and Keck that allow further advances in science. Similarly, we hope that the GMT, and other 30m-class telescopes, will work with the data from the JWST to produce spectacular science.
Q: Can I get a job at GMTO?
A: We are always looking for highly motivated and qualified individuals to work with us. Please visit GMTO’s employment page to see our open employment opportunities.
Q: What educational and outreach activities does GMTO have planned? Can I get materials for my classroom?
A: We plan to create classroom materials and school outreach programs as part of our communications strategy in Chile. Stay tuned for more information.
Q: Where is GMTO’s HQ?
A: The US Project Office is at 465 N. Halstead St, Suite 250, Pasadena, CA, 91107. In Santiago, we are at Presidente Riesco 5335. Of. 501, Las Condes, Santiago, CP 7561127.
Q: What permissions are required to use the images, photos, or videos available on the GMTO.org website?
A: Unless otherwise noted, images and video in these galleries are copyright of GMTO Corporation. For publications, we kindly ask that you contact us at firstname.lastname@example.org to discuss materials release arrangements. Images should always be given the credit line: Giant Magellan Telescope – GMTO Corporation.
Q: How do I get more information?
- Height of telescope housing (enclosure): approximately 65m (213 ft) (22 stories)
- Moving weight of telescope: 1,800 tons (3,968,000 lbs)
- Weight of finished outer off-axis mirror segments: 16.5 tons (33,000 lbs)
- Weight of finished center mirror segment: 15 tons (30,000 lbs)
- Diameter of each mirror: 8.4m (28 ft)
- Effective diameter of combined mirrors: 24.5m (80 ft)
- Total collecting area of mirrors: 368m2 (3961 ft2)
- Mounting type: Altitude/Azimuth
- Wavelength sensitivity of telescope: visible, near-infrared and mid-infrared (320-25,000 nm)