Earthbound Observatories
Not every telescope has to leave Earth. Some of the biggest, sharpest, and most important observatories ever built are still sitting on mountaintops, in deserts, and under the ice at the South Pole — and they do things no space telescope can do, because on the ground you can build them enormous, fix them, and upgrade them.
So why go to space at all, if ground works? Because Earth's air is picky. It lets some kinds of light through and blocks others, like windows in a house. Visible light and radio waves get through pretty well — those are the windows. Infrared gets through only in patches. X-rays and gamma rays are blocked almost completely — those windows are shut. That is why space telescopes exist for X-rays and gamma rays, and why ground telescopes rule for visible light and radio.
That is also why this page is grouped the way it is. "Spectrum" and "frequency" are just two ways of saying the same thing about light. Light is a wave. Frequency is how often the wave wiggles in a second. Wavelength is the distance between one peak and the next. High frequency means short wavelength, low frequency means long wavelength. Radio waves are low-frequency, long-wavelength light. Visible light is in the middle. X-rays and gamma rays are high-frequency, short-wavelength light. A telescope is really just a tool tuned to one part of that spectrum — or, for the last section, to something that is not light at all.
The first sections are observatories actually working right now, as of October 2026. Future ones and one famous retired one are kept separate at the end, so they never mix with the working list.
Visible light — eyes like ours, only much bigger
These telescopes catch the same kind of light your eyes catch, just with mirrors metres across instead of pupils millimetres across. A bigger mirror catches more light and sees finer detail, which is why ground telescopes can be so much bigger than space telescopes: nobody has to launch them.
Vera C. Rubin Observatory — Cerro Pachón, Chile, working
Built for: filming the whole southern sky, over and over, for ten years. Its mirror is 8.4 metres across, and its camera is the largest digital camera ever built for astronomy: 3.2 gigapixels, made of 189 separate sensors, cooled to −100°C. Every few nights it photographs the entire visible southern sky.
What it does: that movie of the sky catches anything that moves, brightens, or explodes — asteroids that might come near Earth, supernovae, stars that flare, and the slow bending of galaxy light by dark matter. It is the survey that will map dark matter and dark energy by sheer numbers: billions of galaxies, not just a few.
Picture this: imagine taking a photograph of the whole sky every few nights for ten years, then playing it as a movie. A rock moving against the stars shows up as a dot that creeps across the frames. A star that explodes shows up as a new dot that was not there last week. That is Rubin's job, and no earlier telescope could do it for the whole sky at once.
Public home: Vera C. Rubin Observatory.
Keck Observatory — Mauna Kea, Hawaii, working
Built for: sharp visible and infrared pictures from two twin telescopes, Keck I and Keck II, each with a 10-metre mirror made of 36 hexagonal segments fitted together like a honeycomb.
What it proved and helped with: Keck measured the stars orbiting the black hole at the centre of our galaxy, helping prove that black hole is really there, and it has studied planets around other stars and galaxies from the early universe in detail no smaller telescope could reach.
Picture this: a 10-metre mirror catches about 100 times more light than your eye, and with adaptive optics — a flexible mirror that wobbles hundreds of times a second to cancel out the blurring of Earth's air — it can see almost as sharply as a space telescope, from the ground.
Public home: W. M. Keck Observatory.
Very Large Telescope (VLT) — Paranal, Chile, working
Built for: four telescopes, each 8.2 metres across, that can work alone or be combined into an interferometer — a trick that makes them act, for detail, like one telescope as wide as the distance between them.
What it proved and helped with: the VLT tracked stars whipping around Sagittarius A*, the black hole at the Milky Way's centre, for decades, and took the first direct picture of a planet around another star.
Picture this: combine light from telescopes spread over a hundred metres, and you can pick out details as fine as reading a newspaper headline on the Moon — if the Moon had newspapers. That is interferometry, and it is how ground telescopes beat their own size limit.
Public home: ESO Very Large Telescope.
Gemini Observatory — Hawaii and Chile, working
Built for: two 8.1-metre telescopes, Gemini North on Mauna Kea and Gemini South on Cerro Pachón, so the whole sky — north and south — is covered with the same kind of sharp eyes.
What it proved and helped with: Gemini has studied exploding stars, distant galaxies, and the air of planets around other stars, and it can swing to a new target fast when something explodes or a gravitational-wave alert comes in.
Picture this: a telescope in Hawaii and a twin in Chile are like having one observatory with two eyes on opposite sides of the planet. When a supernova goes off in the southern sky, Gemini South is already looking at it; when one goes off in the north, Gemini North is.
Public home: Gemini Observatory.
Subaru Telescope — Mauna Kea, Hawaii, working
Built for: wide, sharp pictures from an 8.2-metre mirror, with a camera called Hyper Suprime-Cam that photographs a patch of sky about nine times the size of the full Moon in a single shot.
What it proved and helped with: Subaru's wide surveys have mapped dark matter by measuring how galaxy shapes are bent, and have found some of the most distant galaxies and exploding stars known.
Picture this: most big telescopes look through a straw. Subaru looks through a wide window. That is why it is so good at surveys: it does not just study one galaxy beautifully, it photographs thousands of them at once and lets astronomers count and measure the whole crowd.
Public home: Subaru Telescope.
Radio and millimetre waves — listening, not looking
Radio waves are light with very long wavelengths — from millimetres to metres — and very low frequencies. Earth's air lets them through easily, so radio telescopes work beautifully from the ground. A radio dish does not take a photograph the way a camera does. It listens for faint radio signals, and many dishes working together can act like one dish as wide as the distance between them. That is why radio observatories are often arrays, not single telescopes.
ALMA — Atacama Desert, Chile, working
Built for: millimetre and submillimetre waves — the cold light given off by gas and dust too cold to glow in visible light. ALMA has 66 antennas on a plateau 5,000 metres up, where the air is thin and dry, spread over up to 16 kilometres.
What it proved and helped with: ALMA has photographed disks of gas and dust around young stars, with dark gaps where new planets are forming, and it was part of the Event Horizon Telescope that took the first pictures of black holes.
Picture this: a star being born is hidden inside a cold, dark cloud. Visible light cannot get out. Millimetre waves can. ALMA sees through the cloud the way a thermal camera sees heat through smoke, and shows the baby planetary system taking shape inside.
Public home: ALMA Observatory.
Karl G. Jansky Very Large Array (VLA) — New Mexico, working
Built for: 27 radio dishes, each 25 metres across, arranged in a Y shape on rails, so they can be moved closer together for wide views or farther apart for fine detail.
What it proved and helped with: the VLA has mapped galaxies, black-hole jets, and the gas between stars in radio light, and it produced some of the most famous radio pictures of the sky ever made.
Picture this: slide the dishes out to the ends of the Y, 36 kilometres apart, and the array sees radio detail as fine as a single dish 36 kilometres wide would — a dish nobody could ever build. Slide them back in, and it sees faint, wide radio glow instead. Same telescopes, two different eyes, just by moving them on rails.
Public home: National Radio Astronomy Observatory — VLA.
Five-hundred-metre Aperture Spherical Telescope (FAST) — Guizhou, China, working
Built for: the largest single radio dish in the world, 500 metres across, built into a natural hollow in the hills. It is so sensitive it can catch radio signals far too faint for smaller dishes.
What it proved and helped with: FAST has found hundreds of new pulsars — spinning neutron stars that sweep radio beams past Earth like lighthouses — and it listens for fast radio bursts, millisecond flashes from far across the universe whose cause is still being worked out.
Picture this: a dish 500 metres across is wider than five football fields. It does not move as a whole; instead, panels in its surface shift shape and the receiver cabin moves on cables above it, so it can point at different parts of the sky while the dish itself stays in its hollow.
Public home: FAST — Five-hundred-metre Aperture Spherical Telescope.
Green Bank Observatory — West Virginia, working
Built for: a 100-metre radio dish, the largest fully steerable radio telescope in the world, sitting in the National Radio Quiet Zone, where phones, Wi-Fi, and other radio noise are strictly limited so the telescope can hear faint cosmic signals.
What it proved and helped with: Green Bank has found new molecules in space, studied pulsars, and mapped hydrogen gas in our galaxy and others. It also listens, as part of Breakthrough Listen, for any radio signals that might not be natural.
Picture this: a radio telescope is trying to hear a whisper from across the universe. A mobile phone nearby is like someone shouting in the same room. Green Bank's quiet zone is the rule that keeps the room quiet — no shouting allowed, so the whisper can be heard.
Public home: Green Bank Observatory.
LOFAR — Europe, working
Built for: very low-frequency radio waves, using thousands of simple antennas spread across Europe instead of big dishes. There is nothing to point — the antennas listen in all directions, and computers combine their signals to aim the telescope in software.
What it proved and helped with: LOFAR has mapped the low-frequency sky in detail never seen before, studied the early universe, solar storms, and cosmic rays, and shown that a telescope can be built mostly out of software.
Picture this: a normal telescope points by moving metal. LOFAR points by doing maths. Delay one antenna's signal by a fraction of a second, add them all together, and the combined signal behaves exactly as if the whole array had been pointed at one spot in the sky. Change the delays, and you point somewhere else — instantly, with no moving parts.
Public home: LOFAR.
Event Horizon Telescope — a planet-sized telescope, working
Built for: not one observatory, but a network: radio telescopes around the world — including ALMA, telescopes in Hawaii, Mexico, Spain, Antarctica, and more — all observing the same black hole at the same time, at the same millimetre wavelength.
What it proved and helped with: in 2019 it produced the first ever picture of a black hole, in the galaxy M87. In 2022 it pictured Sagittarius A*, the black hole at the centre of our own galaxy. Both pictures show a glowing ring around a dark shadow — exactly the shape Einstein's theory predicted.
Picture this: combine telescopes on different continents, and you get a telescope as wide as the Earth itself. That is the only way to see something as small, from here, as a black hole's shadow: an orange on the surface of the Moon, seen from Earth, is about the same size in the sky.
Public home: Event Horizon Telescope.
Not light at all — gravitational waves, neutrinos, and cosmic rays
Some observatories do not catch light of any kind. They catch ripples in space itself, ghost particles that pass through almost everything, or particles from space carrying more energy than anything humans can make. These are the "other messengers" from the fields page, and almost all of them work from the ground, because they need huge detectors — kilometres of tunnel, a cubic kilometre of ice, or thousands of square kilometres of desert — that no spacecraft could carry.
LIGO, Virgo, and KAGRA — gravitational-wave detectors, working
Built for: ripples in spacetime itself, made when black holes or neutron stars collide. LIGO has two sites in the United States (Washington and Louisiana), Virgo is in Italy, and KAGRA is underground in Japan. Each one is an L-shaped tunnel with arms kilometres long — 4 kilometres for LIGO — with laser light bouncing back and forth inside.
What it proved and helped with: in 2015, LIGO made the first direct detection of gravitational waves, from two black holes merging 1.3 billion light-years away. Since then the network has detected dozens of mergers, including neutron stars colliding — an event also seen in light, which let astronomers watch the same explosion in two completely different ways.
Picture this: a passing gravitational wave stretches one arm of the detector and squeezes the other, by less than the width of a proton. The laser measures that change. It is like measuring the distance to the nearest star to the width of a human hair — and doing it while trucks drive past, earthquakes rumble, and the ground itself trembles.
IceCube Neutrino Observatory — South Pole, working
Built for: neutrinos, nearly massless particles that pass through planets as if they were not there. IceCube is a cubic kilometre of Antarctic ice, a kilometre below the surface, threaded with more than 5,000 light sensors on 86 strings.
What it proved and helped with: when a rare neutrino does hit an atom in the ice, it makes a tiny flash of blue light, and the sensors catch it. IceCube has traced high-energy neutrinos back to a distant blazing galaxy, proving that some cosmic neutrinos come from outside our galaxy, and it watches the Sun and supernovae for neutrino bursts.
Picture this: trillions of neutrinos pass through your body every second, and you never feel them. To catch just a handful, you need a detector the size of a mountain of ice, buried under a kilometre of more ice to block everything else, at the coldest, darkest, quietest place on Earth.
Public home: IceCube Neutrino Observatory.
Pierre Auger Observatory — Argentina, working
Built for: the highest-energy particles in the universe — cosmic rays, single atomic nuclei carrying as much energy as a well-hit tennis ball, but packed into one particle. Auger covers 3,000 square kilometres of Argentine pampa with 1,660 water tanks and 27 fluorescence telescopes.
What it proved and helped with: when a cosmic ray hits the top of the atmosphere, it smashes into air molecules and makes a shower of millions of secondary particles. Auger's tanks catch that shower on the ground, and its telescopes watch the faint ultraviolet glow it makes in the air. Together they measure the particle's energy and direction, and Auger has shown that the most energetic cosmic rays come from outside our galaxy.
Picture this: the detector is bigger than some countries' national parks, because these particles are so rare — about one per square kilometre per century at the very highest energies. You do not catch them with a small telescope. You catch them by covering an enormous area and waiting.
Public home: Pierre Auger Observatory.
Gamma-ray telescopes that look at the air, not the sky — H.E.S.S., MAGIC, VERITAS, working
Built for: gamma rays too energetic for space telescopes to catch in large numbers. These telescopes never see the gamma ray itself. When a gamma ray hits the atmosphere, it makes a shower of particles that gives off a faint blue flash, lasting billionths of a second. Big mirrors on the ground catch that flash.
What they proved and helped with: H.E.S.S. in Namibia, MAGIC on La Palma, and VERITAS in Arizona have mapped the gamma-ray sky at the highest energies — supernova remnants, pulsars, and the centres of galaxies — and shown which of those objects can accelerate particles to energies far beyond anything on Earth.
Picture this: it is like studying lightning by photographing the flash it makes in the clouds, instead of trying to catch the bolt itself. The atmosphere is not in the way here — it is the detector.
Planned and building — not working yet
These are kept separate, like the planned space telescopes: they are real projects, but their dates can slip, and they have no discoveries yet to picture.
Extremely Large Telescope (ELT) — Chile, building
Built for: a 39-metre mirror — the largest optical telescope ever built — on Cerro Armazones in Chile. It will be able to see planets around other stars directly, and study the first galaxies in detail no current ground telescope can reach.
Public home: ESO Extremely Large Telescope.
Giant Magellan Telescope — Chile, building
Built for: seven 8.4-metre mirrors working as one, giving the light-gathering power of a 25.4-metre telescope, on Las Campanas in Chile, for planets, galaxies, and dark matter.
Public home: Giant Magellan Telescope.
Square Kilometre Array Observatory (SKA) — Australia and South Africa, building
Built for: thousands of radio dishes in South Africa and hundreds of thousands of antennas in Australia, working as one radio observatory with a total collecting area of about a square kilometre — hence the name. It will map hydrogen in the early universe, test Einstein's gravity with pulsars, and search for faint radio signals no current array can hear.
Public home: Square Kilometre Array Observatory.
Cherenkov Telescope Array Observatory (CTAO) — Chile and La Palma, building
Built for: more than 60 gamma-ray telescopes across two sites, catching the atmospheric flashes described above, ten times more sensitively than H.E.S.S., MAGIC, and VERITAS can today.
Public home: Cherenkov Telescope Array Observatory.
Retired, but still famous — not counted as working
Arecibo Observatory — Puerto Rico, 1963–2020, collapsed, not rebuilt
Built for: a 305-metre radio dish, the largest single dish in the world for more than 50 years, built into a natural sinkhole. It was also a radar telescope: it could bounce radio waves off planets, moons, and asteroids and listen for the echo, measuring their shapes and orbits.
What it proved and helped with: Arecibo found the first planets ever discovered around another star — not around a normal star, but around a pulsar, in 1992. It sent the Arecibo Message toward the stars in 1974, mapped Venus through its clouds with radar, and tracked asteroids that might one day come near Earth. Its cables failed and the dish collapsed in December 2020, and it has not been rebuilt.
Picture this: a dish so big you could fit a whole neighbourhood inside it, hanging in a jungle sinkhole, with a receiver cabin suspended on cables 150 metres above it. When it collapsed, astronomers lost not just a telescope, but a radar eye that could actually ping an asteroid and hear it answer.
Ground and space are not rivals. They are two halves of the same job: ground telescopes are huge, repairable, and brilliant at visible light, radio, and other messengers, while space telescopes see the kinds of light Earth's air blocks. The full picture needs both — and the Space Telescopes page is the other half.
Next step if you want it: the same treatment for the fields page's other messengers, or a combined "where to look" index linking each field to the telescopes, ground and space, that actually do it.