Atacama Large Millimeter/submillimeter Array

ALMA Basics

The Atacama Large Millimeter/submillimeter Array is the most complex astronomical observatory ever built on Earth. Teams from North America, East Asia, and Europe merged projects to develop this breakthrough scientific instrument in northern Chile.

ALMA has opened a new window on the Universe, capturing never-before-seen details about the very first stars and galaxies, probing the heart of our Milky Way Galaxy, and directly imaging the formation of planets.

ALMA is a transformative radio telescope that can study cosmic light that straddles the boundary between radio and infrared. Most objects in the Universe emit this kind of energy, so the ability to detect it has been a driver for astronomers for decades.

Millimeter and submillimeter light is easily absorbed by water vapor in the atmosphere. The dry climate and extreme elevation (5000 meters or 16,500 feet) of the site in the Chilean Atacama Desert provides ALMA with the right conditions for detecting these faint signals from space.

ALMA uses 66 high-precision dish antennas of two sizes: 54 of them are 12 meters across and 12 of them are 7 meters across. The total collecting area of this array is over 71,000 square feet.

The 12-meter antennas can be gently hauled around on the backs of custom-made Antenna Transporters in order to form arrays that are either very tightly packed configurations only 150 meters across or spread out to 16 kilometers across, as shown by the animation at left. More extended arrays give ALMA a zoom lens for finer details, while more compact arrays give better sensitivity for larger, dimmer objects.

In addition to the movable array of 12-m antennas, there is the Atacama Compact Array (ACA) of twelve 7-m antennas and four 12-m antennas that images large-scale structures like giant gas clouds. The ACA has two configurations, one of which is a north-south extension to provide a better coverage of sources that are either very far north in the sky or very far south.

In ALMA’s most compact configurations, the level of detail it can see ranges from 0.7″ at 675 GHz to 4.8″ at 110 GHz. In its most extended configuration, ALMA’s resolutions range from 6 mas at 675 GHz to 37 mas at 110 GHz.

At A Glance

Number of antennas66
Dish sizesFifty-four 12 meter and twelve 7 meter
Antenna weight~100 tons
Total collecting area of array71,000 square feet or 6600 square meters
Number of anetnna pads192
Receiver frequenciesFrom 31 GHz to 950 GHz
Resolution0.2 arcseconds to 0.004 arcseconds
Reconfigurable arrayMinimum of 150 m, maximum of 16 km
PartnersUnited States, Chile, Japan, Europe, Taiwan and Canada
Average annual rainfall2.3in or 58mm
Average daily temperature at site34°F or 1.1°C
Elevation16,500 ft or 5000 m

Do you wish you could visit ALMA?

Isolated on the Chajnantor plateau at 16,000 feet (5,000 m) in Chile, ALMA is not easy to get to, but you can get a personal tour here without even a passport.

Take A virtual tour!

ALMA Science

The Atacama Large Millimeter/submillimeter Array is the precision multitool for mapping the once hidden, detailed activities of the Cosmos.

ALMA is a premier telescope for studying the first stars and galaxies that emerged from the cosmic “dark ages” billions of years ago. We find them at great cosmic distances, with most of their light stretched out to millimeter and submillimeter wavelengths by the expansion of the Universe.

In the more nearby Universe, ALMA provides an unprecedented ability to study the processes of star and planet formation. Unimpeded by the dust that obscures visible-light observations, ALMA reveals the details of young, still-forming stars, and shows young planets still in the process of developing.

Using the Universe as a giant chemistry laboratory, ALMA allows scientists to learn in detail about the complex molecules of the giant clouds of gas and dust that spawn stars and planetary systems.

Many other astronomical specialties benefit from the new capabilities of ALMA, such as:

  • Mapping gas and dust in the Milky Way and other galaxies.
  • Investigating ordinary stars
  • Analyzing gas from an erupting volcano on Jupiter’s moon, Io.
  • Studying the origin of the solar wind.
The Horsehead Nebula at different wavelengths.

Here are parallel pictures of the Horsehead Nebula in the optical / infrared / radio wavelengths.  In the optical, dust obscures star-forming activity. In the infrared, the hot, thin layer of dust around the cloud glows. At radio wavelengths, both dust and molecules glow, providing a wealth of information on regions that are otherwise invisible in the optical range.

ALMA Deep Field
Most of the galaxies that are detected in sensitive ALMA images have large redshifts, meaning that they are very very far away from us. This is illustrated in the top row of the image below. It shows the number of low redshift (z<1.5) and high redshift (z>1.5) galaxies expected from a simulated deep ALMA observation. Although the high redshift galaxies are more distant, much more of the dominant emission from warm dust is redshifted into the ALMA frequency bands.

The bottom row shows that with an optical image, such as the Hubble Deep Field, most of the detections are of galaxies with z Star and Planet Formation.

Star formation is the tracer of structure and history in galaxies. Stars form where there is enough gas and dust to make them, so they show us the clumpiness of a galaxy. Big stars explode and leave a buildup of the heavy elements responsible for the creation of the planetary environments in which life in the Universe has become possible.

We know that star formation involves gravitational collapse, but the flow of gas that forms a new star had yet to be found before ALMA came online.

Further, ALMA’s excellent mapping precision allows astronomers to study the characteristics of parent molecular clouds from which stars form. Its sensitivity, angular and velocity resolution, and high frequency performance allows the study of smaller structures, including protostellar fragments, outflows, and disks.

Detecting Extrasolar Planets with ALMA

In order to answer the most basic questions about planetary systems, such as their origin, their evolution, and how common they are in the Universe, scientists need to find and study many more planets around other people’s suns. However, detecting planets circling other stars light-years away is a particularly difficult task.

As soon as it came online, ALMA began providing valuable information about these so-called “extrasolar” planetary systems at all stages of their evolution.

Millimeter/submillimeter-wave telescope arrays such as ALMA can see more detail than current optical or infrared telescopes. The longer waves it detects are not scattered or reflected by interplanetary dust, either in the extrasolar system or our own Solar System. Another important advantage is that, at millimeter and submillimeter wavelengths, the star is not glaring and overwhelming our view of its potential planets as it does in shorter wavelengths. While the star is still brighter than a planet, the difference in brightness between the two is far less in millimeter radiation.

ALMA can see planetary systems in the earliest stages of their formation. It will also be able to detect many more young, low-mass stellar systems and determine if they have the disks from which planetary systems are formed. In addition, ALMA can examine the properties of these disks in detail, including their size, temperature, dust density, and chemistry.

Aging Stars and Dust

Winds of charged gas and particles from aging, cooler stars are the “starstuff” from which Earth and we were formed. The grains shine in the far infrared wavelengths through to ALMA’s millimeter wavelengths. ALMA will see the dusty zone around all giant stars within a few hundred light-years away from Earth.

ALMA has made observation of the shells of gas and dust coughed off by these aging stars, giving us details about the final years of their evolution into white dwarfs and planetary nebulae. Measurements of the shell masses of a large number of planetary nebulae, their brightness and movement will help astronomers better understand the recipe needed to make them.

The diameters of these bloated aged stars can be so huge that if you popped one in place of our Sun, it would take up the entire inner Solar System out to Jupiter. ALMA can image these stars even well beyond the distance to the Galactic center. Measurements of distances to a large numbers of these objects will map the dust producing factories in the Galaxy.


The first molecule discovered in space was helium in 1868 in an optical absorption spectrum taken of the Sun. In 1963, radio telescopes began picking out molecules in space, starting with the hydroxyl radical.

Interstellar gas and dust are concentrated into large regions known as molecular clouds, the birthplaces of new stars, including our Sun, and their planets. This is an amazingly long process, because even the most dense of these clouds is nearly a vacuum by laboratory standards — atoms rarely collide in them. Their low temperatures (10-50 K) mean that few of these rare collisions can even lead to chemical reactions. It is incredible to think that such a vastly empty cloud created everything we’ve ever known on Earth.

Molecular hydrogen, the most abundant gas molecule in space, is formed when two hydrogen atoms stick to the surface of a dust grain and diffuse until they merge into a molecule. When these molecules collide with other molecules, they get knocked into spins. The spin agitates the molecules’ electrons, which emit specific wavelengths of radiation, typically in the millimeter or submillimeter wavelength range. If the molecules are hit hard enough for the bonds between their atoms to bend, then the radiation given off by their wobbling is at infrared levels.

After molecular hydrogen the most abundant molecule we find is carbon monoxide (CO), which astronomers use to map out interstellar clouds in nearby as well as in distant galaxies. More than 180 different kinds of molecules have been found in space, ranging in size from a joined pair of atoms like molecular hydrogen to molecules made from thirteen atoms bonded together.

Most of the molecules are organic (carbon-containing), including ones similar to Earth-like molecules but still others that are a strange assortment of species that could never be stable on Earth. There is also evidence for much larger molecules such as polycyclic aromatic hydrocarbons, which resemble the soot from automobile emission.

The Sun

This ALMA image of an enormous sunspot was taken on 18 December 2015 with the Band 6 receiver at a wavelength of 1.25 millimeters. Sunspots are transient features that occur in regions where the Sun’s magnetic field is extremely concentrated and powerful. They are lower in temperature than their surrounding regions, which is why they appear relatively dark in visible light. The ALMA image is essentially a map of temperature differences in a layer of the Sun's atmosphere known as the chromosphere, which lies just above the visible surface of the Sun (the photosphere). The chromosphere is considerably hotter than the photosphere. Understanding the heating and dynamics of the chromosphere are key areas of research that will be addressed by ALMA. Observations at shorter wavelengths probe deeper into the solar chromosphere than longer wavelengths. Hence, band 6 observations map a layer of the chromosphere that is closer to the visible surface of the Sun than band 3 observations.

ALMA can observe a wide variety of phenomena on the Sun:

  • The structure of the quiet solar atmosphere.
  • Coronal holes (where vast solar winds originate because of diverging magnetic fields)
  • Solar active regions
  • Active and quiescent filaments, and
  • Energetic phenomena like filament eruptions and flares.

One of the great mysteries of the Sun is why it has a solar corona, a huge atmosphere of superhot plasma. At the height of the photosphere (the visible surface of the Sun), the temperature is ~5880K. The temperature then decreases with height for several hundred kilometers. But then something amazing occurs: at greater heights, the temperature increases, gradually at first, and then suddenly to ~3 million degrees! ALMA will probe the “temperature minimum” region of the Sun’s lower atmosphere to learn how that structure is maintained.

At ALMA’s submillimeter wavelengths, it should be possible to detect hydrogen and certain ions in the lower atmosphere. These will tell us about the temperature, density, magnetic field strength, and motions in the low solar atmosphere, layers of the atmosphere that are inaccessible by other means.

Event Horizon Telescope

ALMA Webcams

This is a current image from ALMA, taken from a webcam near the center of the Atacama Large Millimeter/submillimeter Array (ALMA), on the Chajnantor Plateau. You can see even more views from ALMA, including a live interactive 360 degree view, on the Alma Webcam page.

ALMA People

The Atacama Large Millimeter/submillimeter Array (ALMA) is an international astronomy facility, a partnership of North America, Europe, and East Asia in cooperation with the Republic of Chile. This globe-spanning alliance employs people from all over the world.

ALMA is funded in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC), in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

ALMA construction and operations are led on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI), on behalf of Europe by the European Southern Observatory, and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

During construction, ALMA employed over 800 people to design, build, repair, maintain, and run the facilities in Chile and in the partner organizations. Many of the construction staff were South American, and their experience with high-altitude industrial work was critical to the success of the observatory.

At the NRAO, our engineers designed and built receiver cartridges as well as the tiny but sophisticated electronics that went inside them. Many of our scientists and engineers spent months living at the ALMA site in Chile to help the telescopes get tested and integrated into the array. A few of our astronomers relocated to Chile to take on leadership positions within ALMA during its construction.

After its March 13, 2013 inauguration, ALMA maintains a few hundred engineers, astronomers, technicians, and administrators in Chile as staff of the JAO. The partner institutions have our own minimal staff in Chile, with more substantially staffed centers for data reduction and technology development in our own nations.

At the NRAO, the North American ALMA Science Center is the hub of software development and ALMA data handling for the North American partners. Our scientists not only use the telescope for astronomical observations, but they also support the success of other North American scientists who are granted time on ALMA by helping them get the most out of their data.

NRAO Role Models

Want to know more about the work done at NRAO? Hear directly from our employees and how they got here in our Role Model Video Series.


Ask radio astronomy engineers about the early conversations they had with astronomers about building a millimeter-wave telescope array, and they will tell you that the astronomers wanted the impossible. Accurately combining high-frequency waves from several dozen dish antennas in the extreme climate of the Chilean Andes had never been attempted – for good reason.

And yet, now ALMA stands as a masterpiece of engineering, the most complex astronomical observatory ever achieved on Earth, thanks to those same engineers who pushed technology into innovations the world had never seen.

What makes ALMA so extraordinary is its manifold of innovative technologies. By the time an observer receives her data from ALMA, its waves have been processed through the innovations and constructions of thousands of skilled people from around the world.


Using the same technical and scientific specifications, each of the three partner nations came up with our own different design for our robotic, armor-clad antennas.

We worked with Vertex, RSI, a company based in Germany but owned by General Dynamics Corporation, to design our 12-meter dish antennas. Their unique features are a spider-web feed support to hold the secondary mirror and an elevation gear that is driven by a track system. Our dishes are bolted together and their backs enclosed to provide crawlspace maintenance over the many years ALMA is expected to operate.

Our European partners worked with the AEM Consortium (Alcatel Alenia Space France, Alcatel Alenia Space Italy, European Industrial Engineering S.r.L., MT Aerospace) to design their 12-meter antennas with a magnetic sweep drive and dishes that are glued together. Their secondary mirror sits on four smooth poles, and their dishes are covered entirely by a conical backing to increase wind resistance and reduce joints that can expand and contract in extreme temperature changes.

Our East Asian partners worked with MELCO (Mitsubishi Electric Corporation) to design a 12-meter antenna with a bolted dish, spider-webbed feed legs, and a magnetic drive elevation gear. They also designed the twelve 7-meter dish antennas in miniature, but with the smooth, four-poled feed legs, that sit on the same pedestal drive bases as their 12-meter cousins.

The surfaces of all of these dishes, to accurately reflect millimeter and submillimeter waves, are smooth to less than the thickness of a human hair. The amazing panels are bolted on and hand adjusted to this accuracy.

Front End
The radio waves from space hit the dish and bounce up to the secondary mirror balanced precisely above. This mirror reflects the waves down into the heart of the telescope, called its Front End. Here sits a large cooler full of the world’s most sensitive receivers.

These receivers were built around the world, with each partner contributing designs and construction. The giant cooler keeps them to nearly the temperature of space — hundreds of degrees below zero — to block their electronics from creating heat that the receivers detect as radio noise.

The receivers can detect frequencies from 31 GHz up to 950 GHz. When ALMA was in its design phase, the specifications for these receivers were beyond what had ever been possible. In operation now, the receivers perform even better than those specifications.

ALMA’s antennas do not work alone. They must function as a whole of up to 66 antennas. Just as your eyes are separated by a certain distance, the antennas in the array are separated by varying distances. Imagine a pair of eyes 10 miles across! For us to make any sense of what ALMA “sees,” we have to process its collected data from its many pairs of “eyes.”

Complex electronics accurately stitch ALMA’s individual wave detections together into one dataset. The first step in this process is to have exact measurements of where and when the antenna picked up its waves. On each antenna is a clock that timestamps the data using a kind of atomic metronome, or rhythm-keeping device, kept near the supercomputer. The timekeeping waves from this central oscillator beam out to each of ALMA’s antennas. Onboard the antennas, a local oscillator injects this timekeeping beat into a microscopic mixer with the waves coming through the receiver, and a mixed-down signal is digitized and sent back along the fiber into the supercomputer.

Inside the supercomputer, at speeds reaching 17 quadrillion mathematical operations every second, every antenna’s signal is paired with every other antenna’s signal. This is called “correlation,” and it is the secret of how all radio telescope arrays achieve their greatness.

The so-called correlator assembles the data into cubes, slices of signal divided by frequency, that can be hundreds of layers thick. Astronomers process those data in a new software package designed by radio astronomers and software developers for ALMA called CASA.