Giant Magellan Telescope: the smallest "giant" may be the world's best

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When it comes to exploring the Universe, most of the publicity gets taken up by space telescopes: the most fragile and expensive tools in our astronomical arsenal. While it’s true that space telescopes are profoundly powerful — and often explore windows that are closed to us down here on the ground, as Earth’s atmosphere makes many classes of observation impossible — the vast majority of what we know about the Universe comes from ground-based observations. There are a slew of advantages that ground-based observatories possess over their space-based counterparts, including:

the ability to be maintained, cleaned, and repaired,

the ability to be upgraded and outfitted with new instruments,

the ability to be built to larger sizes and greater, heavier weights,

and unfettered access to all of humanity’s ground-based infrastructure,

which explains why plenty of today’s cutting-edge ground-based observatories are decades old, while only Hubble (which itself was serviced multiple times in-orbit) remains relevant among 20th century space telescopes.

There are presently three great ground-based projects attempting to take us beyond the current frontier of 10-meter (technically, 6.5 meter to 10.4 meter ) class telescopes:

the European Extremely Large Telescope, at 39 meters,

the Thirty Meter Telescope, at (as advertised) 30 meters,

and the Giant Magellan Telescope, at 25 meters.

While many people erroneously assume that the largest of these three telescopes will ultimately turn out to yield the greatest scientific results, it’s the smallest one, the Giant Magellan Telescope, that might wind up not only reaping the greatest rewards, but cumulatively for the lowest costs overall. Here’s why.

This photograph shows the Large Magellanic Cloud shining brilliantly in the dark skies provided in the region surrounding Las Campanas Observatory: the future home of the Giant Magellan Telescope. This location is one of the best in the world for ground-based astronomy: with clear skies, high altitudes, dry and still air, excellent seeing, and an astronomy-friendly latitude of 29 degrees south.

Credit : Yuri Beletsky

When it comes to making a telescope, there are all sorts of decisions that need to get made that affect the overall outcome of the telescope.

What site will we choose for it, at what elevation, latitude, and how accessible is the surrounding infrastructure and how supportive is the local community of the project?

What will the optical design of the telescope be, including the mirror size, number of reflections, focal length, and overall telescope architecture?

What types of instruments will make the most of that light that the telescope collects? The choice of instrument, more than any other aspect, impacts the types of science that can be conducted.

What sort of corrections will be made for the atmosphere? Will there be adaptive optics installed, and what flavor of adaptive optics system will be chosen?

What partners will contribute to and invest in the construction and maintenance of the telescope? Usually, there’s a tight relationship between partner investments and the “reward” of guaranteed observing time, determining which institutions, nations, and organizations get to take advantage of the telescope’s capabilities.

And once the data is collected, what is the pipeline for reducing (properly calibrating) that data, making that data first available to the researchers and then to the general public, and for storing and making that data usably searchable to the greater community?

In other words, a telescope is a lot more than just a big mirror; it’s a great suite of things, together, that go into making a successful observatory.

This photograph represents one of the seven high-quality mirror blanks cast for the Giant Magellan Telescope at the Richard F. Caris Mirror Lab at Steward Observatory at the University of Arizona. These 8.4 meter mirror blanks require no new technology or additional specifications over mirrors that have been cast at this lab previously, and offer the largest high-quality, high-precision telescope mirrors ever produced anywhere in the world.

Credit : Ray Bertram/Steward Observatory/University of Arizona

As far as the optical architecture of GMT goes, however, it’s truly remarkable. The largest single mirror we’re capable of making today is just over 8 meters in diameter, and is manufactured in only one place: at the Richard F. Caris Mirror Laboratory at the University of Arizona, where mirrors up to 8.4 meters in diameter are fabricated. Earlier in the 2000s, two of these mirrors were mounted together on the same telescope mount to create the Large Binocular Telescope Observatory , which doesn’t just provide exquisite resolution, but serves as a technology demonstrator for the Giant Magellan Telescope, where seven such mirrors will be tiled together.

The large individual mirror segments each get their own secondary mirror, which reflect the light from the primary through a hole in the central mirror: enabling that light to then go directly to its wide-field instruments. (A third reflection is necessary to send the light into narrow-field instruments.) One often-overlooked fact is that with each mirror reflection, somewhere around 10% of your light is lost. Compared to the larger ELT (with five reflections) or TMT (whose architecture requires seven reflections at minimum , including the adaptive optics system), the two-reflection GMT loses far less light than either of its contemporaries, meaning that despite its smaller collecting area , its greater efficiency translates into roughly the same total amount of useful light reaching the instruments.

This animation shows the light path to science instruments from the incoming starlight using the Giant Magellan Telescope’s architecture. Just two reflections allow the 25.4 meter telescope to keep approximately 81% of the incoming light, while the five-reflection ELT, despite being 39 meters in diameter, keeps only 59% of its light, rendering the effective light-gathering power of the two telescopes comparable, despite their vastly different sizes.

Credit : Damien Jemison, Giant Magellan Telescope – GMTO Corporation

Because of how the light gets focused from GMT’s architecture — particularly for the wide-field instruments — it inherently comes along with a wider field-of-view than either the ELT or TMT. The key is in how quickly the GMT’s optics concentrate the light, leading to what astronomers call a “plate scale” that corresponds to a larger angular size for how much of the sky it can view at once.

For the sake of many types of science, we often need to observe a large number of key objects and/or features across wide areas on the sky. Lensing studies, cluster studies, multi-object spectroscopic studies and more all require imaging of wide areas deeply and at high resolution, and for those endeavors, GMT far surpasses the other 30 meter-class telescopes. The combination of the GMT’s field of view, image quality, and light sensitivity translates into the creation of wide-field images that are:

up to 13 times larger,

up to 16 times faster,

and up to 50% sharper,

than the two other extremely large telescopes currently in development. For studying large populations of stars and galaxies, the GMT will prove itself to be a far superior tool than any of the other telescopes in its class.

Other ELTs need to take as many as 13x more images to capture the same view of the Universe. With the Many Instrument Fiber System, or MANIFEST, the Giant Magellan Telescope will be able to concurrently observe 100’s of objects and increase the number of targets that can be studied at once. MANIFEST is a multi-object fiber positioning system that allows the telescope to feed light from 100’s of celestial objects simultaneously to its spectrographs.

Credit : Damien Jemison, Giant Magellan Telescope – GMTO Corporation

There’s also a huge bonus to the GMT’s architecture over the architecture of either the ELT or the TMT: its smaller number of larger, curved mirrors, as opposed to a larger number of smaller, segmented (hexagonal) mirrors. If you’re observing a distant source of light in the Universe, particularly if it’s a point-like source (e.g., a star, a quasar, an exoplanet, etc.), the ideal case for your telescope is that point-like source makes a point-like image in your instruments.

For segmented, angled mirrors, that’s impossible; the shape you wind up with is more snowflake-like, which is why JWST’s images of bright sources often have large amounts of distortion around them. In addition, there’s also the fact that struts are required to hold the secondary mirror in place, and for most telescopes — JWST, the ELT, the TMT, and even Hubble — those struts block part of the light that reaches and/or reflects off of the primary mirrors. That leads to large “spikes” in the resultant image: diffraction spikes.

However, the spaces between the circular mirror segments of the GMT, by design, are the exact locations where those support struts will be placed. As a result, there will be no diffraction spikes in GMT images , and their images of point-like objects will be the most point-like of any large telescope: an improvement due directly to its ingenious design.

The 25-meter Giant Magellan Telescope is currently under construction and will be the greatest new ground-based observatory on Earth. The spider arms, seen holding the secondary mirror in place, are specially designed so that their line-of-sight falls directly between the narrow gaps in the GMT mirrors, creating a view of the Universe without sharp corners to its mirrors or diffraction spikes around its stars. As one of the two US Extremely Large Telescopes proposed by astronomers and currently in development, it is an essential part of bringing about a new generation in cutting-edge ground-based astronomy facilities.

Credit : Giant Magellan Telescope/GMTO Corporation

In the summer of 2025, the GMT was officially advanced to a phase known as final design, where the last decisions get made about many details of the observatory, including the specifics of its first-generation instrument suite. The final design review is slated for the summer of 2027, with a mid-point review scheduled for September of 2026. One goal of this phase is to complete the final design of the first science instruments that the GMT will be outfitted with, and they include:

GMT-NIRS, which is a high-resolution infrared spectrograph . It will cover the entire (usable, from the ground) near-infrared spectrum with high resolution. The first prototypes of the instruments that will focus the light onto the detectors have already been delivered.

G-CLEF, which will be a Large Earth-Finder instrument , enabling us to make the best-ever radial velocity (stellar wobble) measurements of stars, revealing the masses and orbital periods of any close-in planets around those stars.

GMACS, a multi-object astronomical and cosmological spectrograph , designed for surveys,

and GMTIFS, an integral field spectrograph designed to study the “molecular soup” found within galaxies, including star-forming galaxies and galaxies with active black holes, and will be the first GMT instrument to take full advantage of its adaptive optics technology.

While there will certainly be science cases where the ELT and/or TMT can outperform GMT, it’s vital to be aware that the converse is also true: despite it smaller size, there are many science cases where GMT will be the top-performing telescope out of the three.

This simulated view of a dense star cluster shows how JWST would see it (left) and how GMT would see it (right). Owing to GMT’s unique architecture, there are no diffraction spikes present: just a faint series of concentric circles around the brightest stars in the field. For comparison, every point-like source in JWST imagery comes along with a shape lovingly called a “nightmare snowflake” by JWST scientists: owing to JWST’s segmented hexagonal mirrors, honeycomb-like design, and support struts that block a portion of the light. On all of those fronts, GMT is superior.

Credit : Giant Magellan Telescope – GMTO Corporation

There’s also a huge benefit to the GMT that’s rarely talked about in comparison with the other large telescopes in the 30 meter class: instrument size. Scientific instruments can come in a variety of sizes, and for the 30 meter telescope class, the size of the instruments are related to how quickly the telescope’s optics concentrate the light: more severe light concentration enables smaller instruments to do the same job, while less severe light concentration leads to larger instruments.

The GMT has the fastest, quickest, most severe light concentration of any of the three 30 meter class telescopes, and that translates into it needing the smallest instruments of the three. This impacts:

the cost of the instrument, as an instrument that’s twice the length, twice the width, and twice the depth of a comparable, more compact one will typically wind up costing eight times as much,

and the time to manufacture and calibrate the instrument, which more than doubles (but doesn’t quite octuple) for an instrument that’s twice the length, width, and depth of a comparable, more compact one.

This, importantly, is not a cost that you pay only once, when you build your first-generation suite of instruments, but vastly impacts upgradeability, as future generations of instruments will all be subject to this same penalty. On these metrics, the GMT again outshines all of its competitors by a significant margin: something you’ll never see included in the ELT’s cost accounting because of how the European system sweeps those costs under the rug .

These renderings compare similar science instruments that the ELT, TMT, and GMT are all planned to be outfitted with. Compared with the largest telescope’s (the ELT’s) instruments, GMT’s comparable instruments are three times smaller in each of three dimensions: a factor of 27 different in volume, mass, and likely cost and time to complete construction as well. Despite this size reduction, there is no compromise in the instrument’s performance.

Credit : Damien Jemison, Giant Magellan Telescope – GMTO Corporation

From a scientific perspective, there’s also an incredible opportunity that will come online when GMT begins its science operations: the ability to work synergistically with telescopes like the Vera Rubin Observatory. The basic idea is this:

because of its rapid surveying capabilities, the Vera Rubin Observatory will detect sources from anywhere in the sky that have appeared to change over time,

and then GMT can follow up, swiftly and with incredibly high resolution and sensitivity, to observe whatever source might be in the process of changing,

enabling us to not only discovery what happened (supernova, black hole activity, a tidal disruption event, etc.), but to characterize and understand what occurred,

with the ability to deliver sharper images — with the advent of adaptive optics — than even JWST can.

This has tantalizing implications for a lot of different science cases. Perhaps “little red dots” that exhibit changes could be resolved to be more than just a dot. Perhaps exoplanets could have the molecular contents of their atmospheres identified. For super-Earth classes of exoplanets, for example, GMT expects to be able, through its spectroscopic power, to directly measure:

the temperature of the atmospheric gas,

the pressure where the different species of molecules arise,

the presence or absence of winds,

whether rare variants of these molecules exist, potentially shedding light on their formation,

different isotopes to learn something about the origin of the gas,

all of which enable us to reconstruct the astrochemical history and formation environment of these worlds. In terms of what astronomers call “resolving power,” GMT beats what JWST can do by approximately a factor of 20. Exoplanet science, as a result of the boon that GMT will bring, will get an enormous boost.

When an exoplanet transits in front of its parent star, relative to our line-of-sight, the star’s light doesn’t just get blocked by the planet’s disk, but the planet’s atmosphere filters the star’s light, enabling us to detect those atmospheric signatures by performing transit spectroscopy. While the best transit spectroscopy, today, comes from JWST, with a resolving power of approximately 4000, the Giant Magellan Telescopes resolving power will be more like 80,000, enabling us to detect numerous features and properties that even JWST cannot.

Credit : NASA, ESA, CSA, and STScI

GMT will also allow us, for the first time, to extend adaptive optics into the visible light regime, not just the near-infrared. Remember that a telescope’s resolution isn’t just related to its size, but the number of wavelengths of light that fit across its primary mirror. If GMT can do visible light adaptive optics where TMT and ELT can only do adaptive optics in the near-infrared, the GMT, despite its smaller size, will likely “win” in terms of overall resolution.

Of course, not everyone will benefit from this telescope equally: the biggest winners will be the biggest investment partners (and the scientists under their umbrellas) in the telescope itself. These include:

FAPESP in Brazil,

Australian National University and Astronomy Australia Limited in Australia,

Academia Sinica in Taiwan,

Korea Astronomy & Space Science Institute in Korea (which has just recently increased its investment significantly ),

the Weizmann Institute of Science in Israel,

and ten research institutes in the USA : Arizona State, the University of Arizona, University of Texas, Texas A&M, the University of Chicago, Northwestern University, Harvard University, the Smithsonian Astrophysical Observatory, Carnegie Science, and MIT.

There is a lot of speculation over whether the National Science Foundation — whose funding has traditionally supported the construction of ground-based flagship astronomical observatories , but none since DKIST’s construction was completed in 2020 — will actually follow through and fund GMT , as planned. If it does, then that will translate into additional guaranteed observing time for US institutions and scientists on GMT. But if not, then that simply creates new opportunities for other potential public and private funding partners: nationally as well as internationally.

This world map shows the 16 institutions that are a part of the international consortium that are advancing the Giant Magellan Telescope. Cumulatively, their investments represent the largest-ever private investment in ground-based astronomy, with many decades and potentially even a century’s worth of cutting-edge science expected to be reaped as the ultimate reward for this up-front investment.

Credit : Damien Jemison, Giant Magellan Telescope – GMTO Corporation

In many ways, building a new flagship facility — something that pushes the frontiers of what’s possible in the endeavor of scientific discovery — is the ultimate adventure from an instrumentation perspective. Unlike a space telescope, as soon as the optics and the first instrument are in place, science operations can begin; it doesn’t need to be completed to begin its useful lifetime. Also, unlike a space telescope, a ground-based facility can be maintained, repaired, or even upgraded wholesale, with a new instrument replacing an older one, at any time.

As far as progress goes, Daniel Jaffe, GMTO President, had the following to say when I interviewed him:

“We have a plan. We have a really sound design. We have a great team going for us. We’re working really well with the NSF together. Our sponsors are are ponying up [what’s required for us to make] progress. It really is something that shows that this country can still do big, complicated, sophisticated things together… we’ve gotten really good receptions in in Congress on both sides of the House, because people really feel strongly that that this is kind of an adventure that we can all go on together.”

It’s not just for a few years or even decades that a 30 meter-class telescope like GMT can deliver world-changing results, but most likely for the rest of the 21st century and potentially even beyond. This observatory, one of only three in its class proposed, is bound to surprise us with what it finds, much like JWST has already surprised us with so much of what it’s found. Those who invest in it today, in the project’s still-early stages, will reap the scientific rewards not only tomorrow, but for generations to come.

Ethan Siegel thanks Rebecca Bernstein, Project Scientist for GMTO, and Daniel Jaffe, President of GMTO, for conducting interviews and conversations here in 2026 that made this piece possible.

This article Why the “smallest” giant telescope may be the world’s best is featured on Big Think .