Landmark South America
Cerro Tololo Inter-American Observatory
Cerro Tololo is a mountain in Chile with many telescopes on top. Astronomers use them to study the night sky!
The Cerro Tololo Inter-American Observatory is a group of telescopes on a mountain in Chile, 2,200 meters above sea level. It operates the Víctor M. Blanco 4-meter Telescope. The first observations were made in 1965, and in all the mountain holds around 40 telescopes.
The Cerro Tololo Inter-American Observatory (CTIO), a program of NSF NOIRLab, sits at 2,200 m about 80 km east of La Serena, Chile. Since its first observations in 1965 it has been, in NOIRLab's words, the principal platform for US astronomical investigation of the southern skies. It operates the Víctor M. Blanco 4-meter Telescope, home of the Dark Energy Camera, and hosts around 40 telescopes in all.
Photo: NOIRLab, CC BY 4.0, via Wikimedia Commons
Discover Cerro Tololo
Cerro Tololo is a mountain in Chile, a country in South America.
On top is a white building with a round dome. There are many other telescopes for looking at stars too.
The Cerro Tololo Inter-American Observatory stands on a mountaintop in Chile, about 500 kilometers north of Santiago and 80 kilometers east of the city of La Serena. It is 2,200 meters above sea level.
The mountain holds around 40 telescopes. Scientists from the United States and around the world use them to study the southern sky.
NOIRLab describes CTIO as a complex of astronomical telescopes and instruments about 500 km north of Santiago and 80 km east of La Serena, at an altitude of 2,200 m. From the first observations in 1965, it says, Cerro Tololo has served as the principal platform for US astronomical investigation of the southern skies.
Besides its own telescopes, Cerro Tololo hosts 11 tenant observatories and research projects, around 40 telescopes in total. CTIO is operated by NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA).
- Where
- Near La Serena, Chile
- Altitude
- 2,200 m
- First observations
- 1965
- Telescopes
- around 40
Leveling a mountaintop
In 1962, people chose this mountain for a new observatory.
Workers made the top of the mountain flat. In 1965, astronomers first looked at the sky from here.
In November 1962, Cerro Tololo was chosen as the site for the observatory. The next year, AURA bought a large property called El Totoral, with the mountain near its center.
In January 1964, workers started leveling the top of the mountain. The first observations were made in 1965, and the observatory was inaugurated in November 1967.
NOIRLab's timeline records that Cerro Tololo was selected in November 1962. In January 1963 AURA bought El Totoral, a property of close to 30,000 hectares with Cerro Tololo near its center; the name means a place where totora, a reed-like plant used for roofing, grows. Leveling of the summit began in January 1964.
The first observations followed in 1965, and the observatory was inaugurated in November 1967. In October 1977 the Chilean government declared the CTIO lands a 'Privileged Scientific Sanctuary', where mining is prohibited without the express consent of the President.
The Blanco telescope
The tall white building holds a telescope with a mirror 4 meters wide.
It is named after an astronomer called Víctor Blanco.
On Cerro Tololo, CTIO operates the Víctor M. Blanco 4-meter Telescope. Its main mirror is 4 meters across. Workers started digging for it in December 1967, and it first looked at the sky in November 1974.
In 1995 it was named after the Puerto Rican astronomer Víctor Manuel Blanco, who had once directed the observatory.
Excavation for the 4-meter telescope began in December 1967, and its prime-focus first light ceremony took place in November 1974. NOIRLab calls it a near twin of the Nicholas U. Mayall 4-meter Telescope on Kitt Peak. In 1995 it was dedicated and named in honor of the Puerto Rican astronomer Víctor Manuel Blanco, who had been appointed CTIO's director in August 1967.
The Blanco carries the Dark Energy Camera (DECam), a high-performance, wide-field CCD imager built for the Dark Energy Survey, which aims to probe the dynamics of the expansion of the Universe.
Telescopes on two mountains
There are many telescopes on the mountain, big and small.
On a mountain nearby is another telescope that the observatory also runs.
Besides the Blanco telescope, CTIO runs the 4.1-meter SOAR Telescope on Cerro Pachón, a mountain next door. Smaller telescopes on Cerro Tololo, 1.5 and 0.9 meters wide, are run by the SMARTS Consortium.
Visitors can book free tours of the Cerro Tololo telescopes. A bus takes them up the mountain, and they can visit the Blanco telescope's observing room to learn how astronomers make their observations.
CTIO also operates the 4.1-meter Southern Astrophysical Research (SOAR) Telescope on the adjacent Cerro Pachón, a partnership between Brazil's science ministry, NOIRLab, the University of North Carolina at Chapel Hill and Michigan State University. The Cerro Tololo 1.5-meter and 0.9-meter telescopes are operated by the SMARTS Consortium with support from CTIO.
NOIRLab offers free scheduled public visits: a bus takes visitors from the access gate on Route 41 to the telescopes, including the Blanco's observing room and its main platform gallery, with a view of the 4-meter primary and 1.3-meter secondary mirrors. Because the site is at 2,200 m, NOIRLab does not recommend the visit for people with hypertension, heart or lung problems.
- Main telescope
- Víctor M. Blanco 4-meter
- Blanco first light
- November 1974
- Inaugurated
- November 1967
- Tenant projects
- 11
Under the Milky Way
In this photo, the telescope building stands under a sky full of stars.
The dark shapes in the sky are clouds of dust in our galaxy.
This picture of the Blanco telescope was released by NOIRLab on 20 January 2021. The dark shapes above it look like storm clouds, but NOIRLab explains that they are dusty areas of the Milky Way that block visible light. The bright glitter around them is thousands of stars.
NOIRLab says the Blanco has few stormy nights at its site near the dry Atacama Desert.
The image 'A Stellar Storm' (CTIO/NOIRLab/NSF/AURA/R. Sparks, released 20 January 2021) shows the Blanco's white building and dome under the Milky Way. NOIRLab's caption explains that the dark, cloud-like structures are dusty areas of the Milky Way that absorb visible light, and that the glittering fringes are thousands of stars, just some of the billions in our galaxy.
A person stands at the left edge of the picture, and low mountains line the horizon.
Amazing facts
-
The dark patches in the sky are clouds of dust, not storm clouds!
In NOIRLab's photo, the dark 'storm clouds' are really dusty parts of the Milky Way that block light from the stars behind them.
NOIRLab explains that the dark cloud-like structures in 'A Stellar Storm' are dusty areas of the Milky Way that absorb visible light.
-
The land around the observatory is named after a plant!
The property around Cerro Tololo is called El Totoral: a place where totora, a reed-like plant used for roofing, grows.
AURA's 1963 purchase, El Totoral, covered close to 30,000 hectares; the name means a place where totora, a reed-like plant used for roofing, grows.
-
Mining is not allowed near the telescopes without special permission!
In 1977 Chile declared the observatory's lands a 'Privileged Scientific Sanctuary', where mining is not allowed without the President's consent.
An October 1977 decree made the CTIO lands a 'Privileged Scientific Sanctuary', prohibiting mining without the express consent of Chile's President.
Words to know
- observatory
- a place with telescopes for studying the sky
- telescope
- an instrument that gathers light to make faraway things look clearer
- astronomer
- a scientist who studies stars, planets and space
- Milky Way
- the galaxy that our Sun and billions of other stars belong to
- first light
- the first time a new telescope is used to look at the sky
- altitude
- how high a place is above sea level
Discovery challenge
-
In which country is Cerro Tololo?
Cerro Tololo stands east of La Serena, in which country?
CTIO sits about 500 km north of Santiago, in which country?
- Chile
- Canada
- China
Show the answer
Chile
Chile!
Chile.
Chile.
-
How high up is the observatory?
At what altitude is the Cerro Tololo observatory?
What altitude does NOIRLab give for CTIO?
- 2,200 meters
- 220 meters
- 22,000 meters
Show the answer
2,200 meters
2,200 meters!
2,200 meters above sea level.
2,200 m (7,200 feet).
-
Who is the big telescope named after?
The 4-meter telescope is named after which astronomer?
In 1995 the 4-meter telescope was dedicated to which Puerto Rican astronomer?
- Víctor M. Blanco
- Galileo Galilei
- Edwin Hubble
Show the answer
Víctor M. Blanco
Víctor Blanco!
Víctor M. Blanco.
Víctor Manuel Blanco.
-
What are the dark shapes in the sky in the photo?
In NOIRLab's photo, what are the dark 'storm clouds' really?
What does NOIRLab say the dark cloud-like structures in the image are?
- Dust in the Milky Way
- Rain clouds
- Smoke
Show the answer
Dust in the Milky Way
Dust in our galaxy!
Dusty areas of the Milky Way.
Dusty areas of the Milky Way that absorb visible light.
-
When did the telescopes first look at the sky?
In which year were the first observations made at Cerro Tololo?
From which year, according to NOIRLab, has Cerro Tololo served as the main US platform for the southern skies?
- 1965
- 1865
- 2005
Show the answer
1965
1965!
1965.
1965.
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For parents & teachers
What children will learn
- Describe what an observatory is and what astronomers do there.
- Describe what a telescope's mirror does.
- Explain what the dark lanes in the Milky Way are.
Recommended ages: 4 to 17 · About 15 minutes
Talk about it
- Why might astronomers want to see the southern sky?
- Why would a country protect the land around an observatory?
- What would you like to look at with a big telescope?
Try at home or in class
- On a clear, dark night, look for the Milky Way and sketch any dark patches you can see.
- Find La Serena and Santiago on a map of Chile and mark where Cerro Tololo would be.
Key vocabulary: observatory, telescope, astronomer, Milky Way, first light, altitude
Where these facts come from
- Wikidata , Wikimedia Foundation (CC0 1.0)
- Cerro Tololo Inter-American Observatory , NSF NOIRLab
- NSF Víctor M. Blanco 4-meter Telescope , NSF NOIRLab
- NOIRLab Timeline , NSF NOIRLab
- Cerro Tololo telescopes, Chile (public visits) , NSF NOIRLab
- A Stellar Storm (iotw2103a) , NSF NOIRLab
- File:A Stellar Storm.jpg , Wikimedia Commons
- Wikimedia Commons , Wikimedia Foundation (Per file (public domain, CC0, CC BY or CC BY-SA); see each media record)
Checked, not yet human-reviewed. Last updated November 1, 2026. Spotted a mistake? Tell us.