Astronomical Heritage Finder


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Full Description (IAU Extended Case Study format):
Windows to the Universe (multiple locations): AURA Observatory, Chile

Identification of the property

Country/State Party 
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Coquimbo Region


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AURA Observatory, Chile

  • Cerro Tololo
  • Cerro Pachón

(Part of Windows to the Universe: Leading Optical Observatories)


Geographical co-ordinates and/or UTM 
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Cerro Tololo: Latitude 30° 10′ 09″ S, longitude 70° 48′ 23″ W, elevation 2240m

Cerro Pachón: Latitude 30° 14′ 27″ S, longitude 70° 44′ 12″ W, elevation 2700m

(for more detailed information see “Accurate Geodetic Coordinates for Observatories on Cerro Tololo and Cerro Pachón”, a Technical Report for CTIO by Eric Mamajek (v3, May 2013).


Maps and plans,
showing boundaries of property and buffer zone
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See Figs AO.1 and AO.2.


Area of property and buffer zone 
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The area of AURA property surrounding Cerro Tololo and Cerro Pachón is 34,491ha (85,227 acres). This property, now known as the El Totoral Reserve, serves as the inner buffer zone for protection against light pollution and mining.

Three northern Regions of Chile—one of which is the Coquimbo Region, where the AURA observatory is sited—are protected to some extent against light pollution by Decreto Supremo 686/98 which was signed into effect in 1999 by the then President of Chile. These Regions serve as outer buffer zones for reducing light pollution within these Regions. (Poorly-designed exterior lighting within a buffer zone of typically 300km radius can affect the purity of the natural night sky.) The Fray Jorge UNESCO Biosphere Reserve, comprising the Fray Jorge, Talinay and Punta del Viento National Parks, is located in the Coquimbo Region, ~100km southwest of the observatory, within the outer buffer zone.



Description of the property 
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General Description

The AURA (Association of Universities for Research in Astronomy) Observatory in Chile comprises two mountain-top groups of telescopes: on Cerro Tololo and Cerro Pachón. 

Cerro Tololo is the site of the first of the various major, inter- national observatories that are now operating in Chile. Attracted by the pristine night skies, the world’s astronomers have since made northern Chile the primary centre for major astronomy research observatories in the southern hemisphere. The wide-field, 4m, Blanco telescope was the largest telescope in the southern hemisphere during the period 1975–1997. Clear, dark skies over the Blanco telescope were crucial to its selection by the two groups who used it to make the initial discovery of the acceleration of the Universe, which was announced in 1998. This discovery, awarded the 2011 Nobel Prize for Physics, was one of the major discoveries made in astrophysics during the second half of the 20th century. There are numerous smaller telescopes on Cerro Tololo, funded mainly by the Small and Moderate Aperture Research Telescope System.

Cerro Pachón is the site of the international 8m Gemini South and 4.2m SOAR telescopes and the future site for the 8.2m, very-wide-field Large Synoptic Survey Telescope.

The El Totoral Reserve, Cerro Tololo and Cerro Pachón

The Cerro Tololo Inter-American Observatory is located about 500km north of Santiago, Chile, about 52km east (80km by road) of La Serena, at an altitude of 2200 meters. It lies near the centre of a 34,491ha (85,227-acre) site known as Estancia El Totoral (Fig. AO.1) which was purchased by AURA on the open market in 1967 for use as an astronomical observatory.

Roughly in the center of the property lies Cerro Tololo on which is located a still-increasing number of working optical astronomical telescopes, the largest of which is the 4m Victor M. Blanco (Figs AO.2–AO.4).

Fig. AO.1: The area of the El Totoral Reserve (Image: NOAO/AURA/NSF)


Fig. AO.2: The summit of Cerro Tololo (Image: NOAO/AURA/NSF)


Fig. AO.3: Cerro Tololo from the east (Photograph: NOAO/AURA/NSF)


Fig. AO.4: Cerro Tololo from the north (Photograph: NOAO/AURA/NSF)


On the southeast side of the property lies Cerro Pachón where the Southern Hemiphere Gemini 8m and the 4.2m SOAR telescopes are located (Figs AO.5a and AO.5b).

Fig. AO.5a (left): In this picture, looking up at the face of Pachón from the northwest, the Gemini dome can be seen when it was under construction. The SOAR site is behind the promontory in the top center of the picture  (Photograph: NOAO/AURA/NSF)

Fig. AO.5b (below): A broader view of Cerro Pachón years later (2011), where SOAR (left) and Gemini (right) can easily be distinguished.  Two bumps further to the right of the Gemini site mark where rock-blasting preparation work on the site for the LSST has since been carried out (Photograph: NOAO/AURA/NSF)


History and development 
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Archaeological/historical/heritage research

The Diaguita and Molle cultures in the immediately surrounding area are extinct. There are two examples of rock art (not necessarily connected with astronomy) on Cerro Pachón. A statistical study of Molle sites might reveal further astronomically relevant information.

CTIO historic highlights

The Cerro Tololo Interamerican Observatory, on the AURA property near La Serena, was the first of the large, modern, international, astrophysical observatories to be set up in Chile. The following is a detailed, decade-by-decade list of historic and development highlights since the late 1950s.


June 1958

Prof. Federico Ruttlant of the U. of Chile visits Yerkes Observatory and proposes a cooperative observatory project to Drs. Kuiper and Hiltner.

July 7, 1958

Dr. Kuiper contacts Dr. Shane to explore possible AURA interest in the cooperative observatory. AURA is not then in a position to consider the project.

Jan 8, 1959

U. of Chicago applies to the U.S. Air Force for funds for a 40-inch telescope in Chile to be located near Santiago. The Air Force expresses interest and agrees to fund site testing program.

May-Jun 1959

Dr. J. Stock, later CTIOs first Director, travels to Chile and with U. of Chile personnel and equipment, a site testing program is organized. The first sites tested were near Farellones and Cerro El Roble.

June 1, 1959

The Universities of Chile, Chicago, and Texas sign an agreement for a cooperative observatory to be funded by the U.S. Air Force. The 40-inch telescope project becomes a 60-inch telescope project. Dr. Clemence suggests the project title: The Inter-American Observatory in Chile.

Aug 19, 1959

Dr. I. Epstein of Columbia U. starts another site testing program in Chile with NSF funding. This program aimed at comparing sites in Chile, Argentina, Australia, and South Africa. A month later Drs. Stock and Epstein coordinated their programs. Eventually, the U. of Columbia and Yale U. established an astrometric observing station near San Juan, Argentina.

Oct 19, 1959

Dr. G. Keller of the NSF expresses the interest of the NSF in supporting the Chilean Observatory project. A policy advisory committee with AURA, NSF, Air Force, and Universities of Chile, Chicago, and Texas representatives is formed to consider the future of the project.

May 25, 1960

AURA is asked to take over construction and operation of a joint Chilean Observatory.

June 30, 1960

AURA assumes reponsibi1ity of site surveys for U.S. observatory in Chile under the auspices of the U.S. Air Force and subsequently the National Science Foundation.


Feb–Aug 1960

Site surveys extended northward to include Tololo, Morado, and other mountains near Vicuña.

August 1961

0.41-m telescope hauled to Cerro Tololo on mule back for tests of site.

December 1961

AURA and the U. of Chile sign an agreement for establishment of the observatory in Chile.

December 1961

CTIOs first administrative office opens at the Chilean National Observatory at Cerro Calán, Santiago.

Oct 11, 1962

Eight hectares lot is purchased in La Serena.

Nov 23, 1962

Cerro Tololo chosen as site and the Cerro Tololo Interamerican Observatory’s current name adopted.

Nov 25, 1962

AURA buys the property El Totoral, 30,000 ha, with Cerro Tololo near its center.

Dec 2, 1962

Traditional flag-raising ceremony held on Tololo in company of Chilean officials who climbed the mountain on horseback.

January 1963

Chilean Congress, with sponsorship of the U. of Chile, approves duty-free importations by AURA. Such importations were to be handled by the U. of Chile.

February 1963

NSF approves the funding of a 0.92-m telescope for CTIO.

April 1963

Dr. J. Stock is appointed first Director of CTIO.

September 1963

First vehicle driven to Tololo on the primitive, but passable, 38-km access road.


Temporary powerhouse, warehouse, and maintenance shops completed, and 25 years later, the temporary structures are still in use.

June 1963

Representatives of ESO and AURA meet to discuss possibility of the European Southern Observatory being located in CTIOs grounds. In 1965 ESO decides on La Silla for its location, further north and closer to the Atacama desert.

Dec 63 – Feb 64

Dr. H. Babcock, director of Mt. Wilson and Palomar Observatories, visits CTIO to initiate a site survey on AURAs grounds for a Carnegie Southern Observatory. After initial tests at Cerro Pachón, further testing was limited to Cerro Morado. Eventually the Carnegie Observatory is established on Las Campanas, a mountain between Cerro Tololo and Cerro La Silla, much closer to the latter.

January 1964

Construction initiated of the Headquarters building in La Serena.

February 1964

First radio messages sent between CTIO and KPNO. The University of Chile allowed CTIO to use its assigned wavelengths and call letters.

March 1964

AURA Board approves five-year master plan for development of CTIO.

May 1964

First 800 books acquired for Library.

Jan-Jun 1964

Leveling of the top of Cerro Tololo carried out.


The U. of Chile, La Serena Branch, on a cost-free basis, allows CTIO to build an access road to CTIOs headquarters across its property.

June 1964

Water being pumped to Tololo from a spring at Los Placeres.

July 1964

Within weeks of leveling the summit of Tololo, housings started for the 0.41m, 0.92m, and 1.5-m telescopes.

June 30, 1965

The CTIO staff consists of seven employees, two of which were stationed in Tucson, Arizona. By January 1976 when the 4-m telescope is put into operation the staff numbered 175, probably close to its historical maximum.

December 1965

First 50,000-gallon water storage tank installed on Tololo.

December 1965

An additional hectare containing a house added to the La Serena compound at its western end (Calle Cisterna).


Five houses completed on Tololo.

Oct 26, 1966

AURA concludes agreement with the University of Michigan to install the Curtis Schmidt telescope on Tololo on a 10-year loan basis; the agreement was extended for 25 years in 1975.

December 1966

Ford Foundation decides to donate $5 million on matching-grant basis with NSF for construction of a 4-m telescope in the Southern Hemisphere.

March 1967

0.92-m telescope acquired and installed outdoors; moved to its permanent housing in Kay.

April 3, 1967

The housing for the Curtis Schmidt telescope is completed.

April 1967

At Punta del Este, Uruguay, U.S. President Johnson and Chilean President Frei jointly announce that the Ford-NSF 4-m telescope would be installed on Cerro Tololo.

May 1967

Housings for the 0.4l-m and 0.92-m telescopes are completed.

September 1967

Previously planned houses on Cerro Tololo for the CTIO Director and a Mountain Superintendent, as well as three other houses, are eliminated from the Master Plan.

October 1967

1.5-m telescope installed.

October 1967

The administrative/scientific (round) office building is completed on Tololo.

October 1967

Astronomers Dormitory and Dining Hall first occupied.

Nov 3, 1967

The U. of Chile and CTIO jointly sponsor a conference on Astrophysical Photometry in Santiago as part of the CTIO inauguration program.

Nov 6, 1967

First light on the 1.5-m telescope.

Nov 7, 1967

Official inauguration of CTIO. The benediction is given by Msgr. Fresno, later Cardinal Fresno. Chilean President Frei visits Tololo.

November 1967

Tololo instrument shop completed; it subsequently becomes the electronic shop, and eventually the visitors’ center.

Late 1967

Late in the year, the decision is made to locate in La Serena all CTIO service shops not needed on Tololo: e.g., the instrument shop, ETS offices and shops, the library, receiving warehouse, main garage and computer center.

December 1967

Excavation started for 4-m telescope housing.

March 1968

An additional eight hectares are added to the La Serena compound at its eastern end (hilltop).


Negotiations initiated to modify importation procedures of CTIO shipments.

July 1968

The first prefabricated houses for U.S. hired

Aug 7, 1968

With AURA approval, the CTIO Director and the Rector of the U. of Chile sign an agreement allowing telescope time for U. of Chile astronomers.

December 1968

The Government of Chile extends to CTIOs U.S. Hires certain benefits enjoyed by foreign employees of the United Nations branch office in Santiago.

March 1969

Lowell 24-inch telescope installed.

May 21, 1969

Harvard and Yale Universities and MIT plan possible installation on Cerro Morado of a 90-inch, a 36-inch, and a l6-inch telescope.

July 25, 1969

The 4-m Cervit mirror blank is cast by Owens-Illinois Company of Toledo, Ohio. The l7-ton casting is the largest casting ever made.

September 1969

Passage by the Chilean Congress of a law modifying importation procedures and freeing CTIO from certain taxations and limitations of its operations.


June 1970

The U. of Chile and AURA award the first jointly-financed fellowship for Chilean graduate students in Astronomy.

March 1971

NASA, the U. of Chile, and the Smithsonian Institution install on Cerro Morado a station to observe barium clouds injected into the upper atmosphere by Germanys Max Planck Institute.

March 1971

The U. of Chile puts into operation a seismograph station on Cerro Tololo.

March 1972

4-m telescope housing completed.

June 1972

Yale University agrees to lend its 1-m telescope to CTIO. The telescope is put into operation one year later.


A low wattage microwave relay station is erected on a side spur of Cerro Tololo by the Chilean Telecommunications Agency, ENTEL, per agreement with AURA.

December 1975

Completion of fine tuning of, and addition of the cassegrain secondary to the 4-m telescope.

Jan 1, 1976

First visiting astronomers use the 4-m telescope.


At the request of CTIO, the Government of Chile declares Cerro Tololo a privileged scientific sanctuary where mining is prohibited without permission of the President of Chile.


November 1982

Columbia University starts operation at CTIO of a 1-m diameter, millimeter radio telescope.

November 1982

The AURA Board of Directors agrees on a reorganization whereby CTIO becomes part of NOAO along with KPNO and the US National Solar Observatory.; NOAO comes into existence officially on February 1, 1984.

January 1986

Dr. Robert Williams is appointed CTIO Director until July 1993.

Feb 23, 1987

Supernova 1987A explodes in the Large Magellanic Cloud (a satellite galaxy of the Milky Way). It is first naked eye supernova in four centuries, and sparks intense investigation with CTIO telescopes (e.g. Phillips et al. 1988).


November 1993

Dr. Malcolm G. Smith is appointed CTIO Director until October 2003.

November 1993

In response to a request for guidance, CTIO receives advice from Senator Edgardo Boeninger that Chile is about to set up a Chilean equivalent of the US Environmental Protection Agency (CONAMA) and recommends putting CTIOs interest in protection against light pollution on its early agenda.


The 1.3m (50-inch) IR survey telescope begins the southern component of the Two Micron All Sky Survey (2MASS), mapping the sky in the near-infrared, with sensitivity limits of J = 15.8, H = 15.1 and K = 14.3. The 2MASS survey produced a catalog of over 470 million infrared point sources (mostly stars), 1.6 million extended sources (mostly galaxies), and helped in detecting hundreds of the nearest substellar objects (brown dwarfs) to the Sun.


President Frei Ruiz Tagle signs the Supreme Decree 686/98, now more normally referred to as the norma luminica, which provides a legal foundation for the effort to protect astronomy in northern Chile. This action probably gained an increase of a couple of decades in the useful future lifetime of the sites in northern Chile for astronomical research.

Dec 18, 1998

Science Magazine recognizes discovery of the accelerating universe as the Science Breakthrough of the Year for 1998. Given the then wide-field capability of the telescope the clear dark site and efforts to maintain pixel-limited imaging performance, much of the early work by the two main groups who made the discovery was carried out on Cerro Tololo at the Blanco 4m telescope. Important calibration work for using Type Ia supernovae as distance indicators was carried out by CTIO staff in Calan/Tololo supernova survey with the Michigan Curtis Schmidt telescope (e.g. Hamuy et al. 1996). In the Riess et al. 1998 study demonstrating the existence of the dark energy, the 10 Type Ia supernovae analyzed with redshifts 0.16 < z 0.62 were all discovered with the prime-focus CCD camera on the Blanco 4-m telescope.

August 1999

Event “The Sun, Our Star” heralds the formation of a local schools network RedLaser and organized local public outreach from Cerro Tololo.

October 1999

The norma luminica Chilean law to protect the future of astronomy in northern Chile (DS 686/98) comes into force.



Michigan State University becomes a partner in the SOAR 4.1m telescope, finalizing the partnership formation process led with tenacity and patience by the University of North Carolina. It took UNC 18 years to get the telescope built after multiple partners had bowed out.


Given the greater medium-term risk of light pollution at Cerro Tololo and Cerro Pachón, the national office for the protection of the skies of northern Chile (OPCC) is set up and the former Regional director of CONAMA is hired as the first director of the OPCC.

March 2002

International Conference on Light Pollution held in La Serena.

November 2003

Dr. Alistair R. Walker is appointed CTIO Director until October 2008.

April 17, 2004

Dedication ceremony for the SOAR 4.1m telescope held on Cerro Pachón.


In response to increasing pressure on CTIO to reduce its support of the small telescopes on Cerro Tololo, the community-led SMARTS consortium - set up a few years earlier to provide balance for the installation of Gemini South and SOAR on Cerro Pachón - begins operation of the SMARTS telescopes (0.9m, 1.3m, 1.5m).

May 17, 2006

El Peñón summit on Cerro Pachón is selected as the future site for the 8.4-m Large Synoptic Survey Telescope (LSST).

November 2008

Dr. Robert C. Smith is appointed CTIO Director until September 2012.



In the context of humanity’s ability to see dark skies in the future and carry out optical observational astronomy from the ground, Cerro Tololo and Cerro Pachón are highlighted as case studies in the IAU/ICOMOS/UNESCO World Heritage Centre book on Heritage Sites of Astronomy and Archeoastronomy as one of five Windows on the Universe (along with dark-sky sites in Hawaii, La Palma, New Zealand and the East Alpine Starlight Reserve).

March 8, 2011

LSST first blast: initiation of site leveling of the El Peñón summit of Cerro Pachón in preparation for the LSST.

Oct 4, 2011

2011 Nobel Prize in Physics won by three astronomers, for the discovery that the expansion of the Universe is speeding up. Saul Perlmutter (Lawrence Berkeley National Lab) led the Supernova Cosmology Project while Brian Schmidt (Australian National University) and Adam Riess (Johns Hopkins/Space Telescope Science Institute) were leading members of the High-z Supernova Search team. Present (Chris Smith) and past (Mark Phillips, Nick Suntzeff, Mario Hamuy, Bob Schommer) CTIO staff members were members of the High-z team. Both teams announced their results in 1998. Both teams used the Blanco 4m telescope and prime focus imagers in the period 1994-1998 for some of their most critical observations. And prior to this, important precursor observations were made on the Curtis Schmidt telescope by Mario Hamuy and Jose Maza (U. Chile). CTIO staff, both scientific and technical, were crucial in providing the support that allowed these very difficult observations to be made successfully. At that time, the Blanco telescope plus Big Throughput Camera was the most powerful CCD camera in the world. The unexpected discovery that the expansion of the universe is speeding up led to the concept of dark energy and that the Universe we see (stars etc) represents only a very minor constituent of the mass-energy budget of the universe.


Commissioning of the Dark Energy Camera (DECam) wide-field imager on the Blanco 4-m telescope, and initiation of the Dark Energy Survey — a comprehensive program to characterize the evolution of the dark energy over cosmological time.

Nov 23, 2012

50th anniversary of Cerro Tololo Inter-American Observatory.



Justification for inscription

Comparative analysis 
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See Windows to the Universe: Leading Optical Observatories (general description)


Integrity and/or authenticity 
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See Windows to the Universe: Leading Optical Observatories (general description)


Criteria under which inscription might be proposed 
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See Windows to the Universe: Leading Optical Observatories (general description)


Suggested statement of OUV 
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See Windows to the Universe: Leading Optical Observatories (general description)


State of conservation and factors affecting the property

Present state of conservation 
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The buildings and telescopes at the AURA observatory are well maintained, consistent with the operation of a major research facility.

Sky brightness caused by the current level of lighting from nearby cities (La Serena, Coquimbo, Ovalle, Andacollo, and Vicuna) is not worrisome at present. According to recent surveys (Kriscuinas et al. 2007; 2010: see especially figs 1 and 3), the broad-band, artificial sky background is, even in the worst directions, still only detectable in broad-band measurements within 10-15 degrees of the horizon, and does not (yet) interfere with any observatory research.


Factors affecting the property 
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Developmental pressures

Of the major observatories in Chile, the AURA Observatory is currently the most threatened by the gradual encroachment of light pollution and therefore serves as a marker for gaining experience over the next two or three decades in how to protect the dark skies not only over this observatory but also over other sites in Northern Chile, including the site of the European Extremely Large Telescope (E-ELT, the largest ground based optical telescope currently planned anywhere in the world).

A CTIO study in 2004 discussed predictions for night sky brightness at Cerro Pachón, in the context of planning for the LSST. It presents the relevant numbers and several projections, depending on population growth and the success of lighting controls. The study demonstrates that with successful lighting awareness campaigns, such as that which CTIO/AURA has launched, Cerro Pachón and Cerro Tololo can continue to be prime astronomical sites for at least 3 decades even under the assumption that the dramatic population growth of the La Serena/Coquimbo conurbation (32% between 1992 and 2002) continues into the future.

That assumption may be proving to be the correct one—the rapid growth over the last 30 years of La Serena and Coquimbo is a concern. The current population is 412,586 (according to preliminary census figures for Chile released in August, 2012) and large mining projects in northern Chile have accelerated the arrival of inhabitants to the conurbation. This 27% growth over the last decade compares fairly closely with the predictions of the constant-growth model used in the CTIO study which, ten years ago, predicted a population of 425,000 for 2012.  Currently 19 infrastructure projects totalling US$277,000,000 are being planned for this conurbation. 

This rapid growth is in the context of half of humanity now living in cities. According to National Geographic Magazine (December 2011 issue, pp. 138–139), in 1800 there were only three cities of one million or more, in 1900 16 such cities and in 2010 442 of them, with 21 having more than 10 million and several exceeding 20 million. Thus the earlier assumptions about city development made in the CTIO study—that the rapid-growth model for La Serena and Coquimbo was unlikely to continue for more than two decades—remain uncertain.

With regard to street lighting, the recent increase in population in La Serena/Coquimbo has increased the number of vehicles. Traffic licences for La Serena/Coquimbo are increasing at a rate of 9.2% each year (compared with Chile’s national rate of increase, which has reached 7.5%). There are currently 90,000 vehicles in La Serena. The Ministry of Transport expects this number to double by the year 2020—about the time that LSST will begin operations. 

It is fortunate indeed that light pollution control was begun in earnest in Chile 20 years ago. It is also particularly fortunate that most of the current conurbation is shielded by a range of coastal mountains which still block a direct view of most of the conurbation from Cerro Tololo and Cerro Pachón.

Pressure continues to develop a bi-oceanic corridor through Argentina, under the Andes (the Agua Negra tunnel) and down the Elqui valley near the AURA observatory - in support of trade between Brazil and China. It is important to learn the lessons provided by Los Angeles plus the Palomar and Lick Observatories in the USA. Los Angeles grew in a hundred years from an urban center about 2/3 the size of the current La Serena-Coquimbo conurbation to a city of 5,000,000 people. Protection of the northern Chilean sites needs continued attention—we cannot afford to be complacent.

Fig. AO.6: The night sky at Cerro Tololo (Photograph: Arturo Gomez/NOAO/AURA/NSF)


Environmental pressures

When the El Totoral area was purchased by AURA, the land supported a number of subsistence farmers and goat herders. They were allowed to continue to live on the reserve after it was purchased by AURA and have gradually been leaving voluntarily for more lucrative jobs in the nearby towns.

As a result of the departure of most of its human inhabitants and a policy combining environmental protection with benign neglect on the part of the Observatory, the property sees little human activity except for the roads and relatively small areas on the tops of Cerro Tololo and Cerro Pachón. As a result, much of the reserve is gradually returning to its natural state. Many native species of plants and animals, long thought in danger of extinction, are now returning. The last half of the trip to Tololo is an excellent opportunity to see a reasonably intact Chilean desert ecosystem. During the first portion of the journey, to a few km beyond El Totoral, the effect on the environment of humans, bad farming practices and the remaining goats is easily seen. That damage will take many years to heal.

Visitor/tourism pressures

Cerro Tololo is open to the public every Saturday, summer and winter, weather permitting. For safety and security reasons, the number of visitors on any given day is limited to two groups of 40 people. One group meets at the gatehouse at 9am and the other at 1pm. Because the number of visitors is limited, advance reservations are essential. During the tourist season from mid-December to March, reservations must be made several weeks in advance. Permits must be picked up in La Serena before proceeding to the mountain. Access to the mountain is by private vehicles only. There is no public transportation.

Permits are free and may be obtained from the Reception Desk in La Serena. Tours are conducted at no charge by a professional guide. Total elapsed time from leaving the gatehouse until returning to the highway is approximately three hours.

AURA has worked with local tourism agencies and municipalities in support of the development of 7 public observatories. This successful program has relieved the pressure on the AURA observatory, which is not open to the public at night, while creating employment and educational opportunities for local people. The Coquimbo Region is now known as La Region Estrella largely as a result of these initiatives.

For more information on public access to CTIO, click here. As the next image shows, not only human visitors are interested in visiting the summit of Cerro Tololo ...

Fig. AO.7: The Cerro Tololo dome reflected in the eye of a condor (Photograph: Arturo Gómez/NOAO/AURA/NSF)


Number of inhabitants

See "Developmental pressures" and "Environmental pressures" above.


Protection and management

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The entire 34,491ha (344.9km²) site is owned by AURA, the Association of Universities for Research in Astronomy.  AURA is recognized by the Chilean Government as an accredited International Organization, with a variety of diplomatic privileges. The stakeholders are the 40 international member institutes of AURA Inc.


Protective designation 
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The El Totoral Reserve around Cerro Tololo and Cerro Pachón has been declared “of Scientific Interest” by the Chilean Government, which protects it from incursion by mining interests.

The Region of Coquimbo is one of three Regions in northern Chile where artificial lighting will be governed by the requirements of Supreme Decree 43/2013—already signed by the President of Chile on 3 May 2013 and which will come into effect on 3 May 2014—as an updated revision of Supreme Decree 686/1998 (the “norma lumínica”). This protects against light pollution.


Means of implementing protective measures 
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Any mining activity within this area, including prospection work, would require the written permission of the President of the Republic of Chile.  The buffer zone is protected from mining operations via a formal program of constant monitoring of requests for mining activity.

On the property, AURA voluntarily complies with and exceeds all environmental protection requirements of the Chilean government, including of course, the norma luminica.

Outside the property, AURA carries out education work via its membership of the consortium which supports the Office for the Protection of the Quality of the Skies of Northern Chile (OPCC) and through activities in local schools. The OPCC has the mission to carry out public education and to assist the Chilean government in the protection of this natural resource.

Under the supervision of the Superintendent’s Office of Electricity and Fuel (SEC, Chile) and local municipalities, about 50% of all street lights in the Region of Coquimbo have now been modified or replaced in order to comply with the requirements of the norma luminica.

The observatory has formed a network of 200+ schools and support organization in collaboration with the Municipality of La Serena, the University of La Serena and the national Libraries, Archives and Museums directorate (DIBAM). The Coquimbo Region has developed an extensive astro-tourism initiative which has flourished because of the contrast between the polluted skies of much of Europe, Japan and the USA and the skies as seen through the Windows to the Universe. Seven public and private observatories have opened in the Region in response to demand from networks of schools and from tourists (see http://www.ctio.noao.edu/AURA/CADIAS/). Recognizing this natural and cultural heritage, the motto of the Coquimbo Region of Chile is now “Coquimbo—the Star Region”.


Existing plans 
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AURA has been working via the OPCC with the Chilean Ministry of the Environment to develop an updated version of the Norma Lumínica. This will limit proliferation of blue-rich Light Emitting Diodes. The blue region of the optical spectrum is currently without significant pollution (Kriscuinas et al. 2010), even from Cerro Tololo looking at the sky directly over the La Serena/Coquimbo conurbation (largely as a result of using sodium lighting fixtures). An additional advance under the new Norma will be to limit the amounts of light allowed and to have external lighting upgraded to full cut-off (full shielding above the horizontal).

The longer-term aim is to have the Norma Lumínica upgraded to a Chilean law, rather than a set of environmental rules.


Visitor facilities and infrastructure 
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The Coquimbo Region has developed an extensive astro-tourism initiative which has flourished because of the contrast between the polluted skies of much of Europe, Japan and the USA and the skies as seen through the Windows to the Universe. Seven public and private observatories have opened in the Region in response to the demand from networks of schools and from tourists: see http://www.ctio.noao.edu/noao/content/astro-tourism-chile. Recognizing this natural and cultural heritage, the motto of the Coquimbo Region of Chile is now “Coquimbo—the Star Region”.



Indicators for measuring state of conservation 
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Fig. AO.8: The setting Zodiacal Light as seen looking SW from Cerro Tololo.

a (above): With the Zodiacal Light still quite high in the sky.

b (below): After the Zodiacal Light has set.

Roger Smith / CTIO)

Measurements of broad-band magnitudes per square arcsecond alongside regular photography provide a first-order guide to the source and extent of light pollution sufficient for most purposes including setting local priorities for protective action. The zodiacal light (sunlight reflected off dust in the plane of our solar system) is easily seen from first-class dark-sky locations, but not from even fairly mildly polluted sites. Major observatories, such as those included in the Windows to the Universe, usually support studies every few years to check the status of light pollution from their surroundings. An effort to introduce international, continuous monitoring on an intercomparable basis at many sites is currently being attempted by the International Dark Sky Association; the Cerro Tololo site has been used for one year as one of three beta test sites for this work, which will take and send measurements taken automatically each night for processing and display on the web, in a graphical form, readily accessible to the public. One plan is to have one detector pointed at the zenith, the other pointed over the source of most serious local light pollution, at each of the Windows to the Universe.

Measuring the night sky brightness in order to quantify contamination in the directions in which we are interested (e.g. zenith distances less than 60 degrees) is difficult at world-class sites owing to the necessity of measuring an effect that may be only 1–2 % of the sky brightness. Additionally, interpretation of the results may not be straightforward. The natural brightness varies on timescale of minutes to years with amplitudes of several tenths of a magnitude (Patat 2006). The choice of sky position is important. Lavasseur-Regourd and Dumont (1980) model the increase in background as the ecliptic plane (and its zodiacal light) is approached. In the V band the effect is 0.3 magnitudes at ecliptic latitude 0 and falls by roughly 0.05 mag for each 10 degree increase in ecliptic latitude.

Two of the most recent monitoring papers for Cerro Tololo are Kriscuinas et al. (2007; 2010). Cerro Tololo is closer to La Serena-Coquimbo and Vicuña than Cerro Pachon and is roughly estimated to suffer 65% more light contamination than Cerro Pachon. It will be necessary to analyze a lot more data before we have any hope of measuring long-term trends in light pollution over Cerro Tololo. It is particularly important to support efforts to fund a second epoch of the World Atlas of the Artificial Night Sky Brightness (Cinzano et al. 2001)—see Figures 9a (the world) and 9b (Chile) reproduced from that Atlas.


Fig. AO.9: Images based on work done for the World Atlas of Artificial Night-Sky Brightness by P. Cinzano, F. Falchi (University of Padova) and C.D Elvidge (NOAA National Geophysical Data Center, Boulder). published in the Monthly Notices of the Royal Astronomical Society in 2001. a (left): Northern Chile. b (below): The world. Reproduced by permission of Oxford University Press


Significant modelling extensions have recently been reported by Cinzano and Falchi (2012). However, more computing power will be needed to include the beneficial screening effect of coastal mountains which protect the summits of Cerro Tololo and Cerro Pachón from a direct view of most of the lights in the La Serena/Coquimbo conurbation. 

Another approach is being followed based on taking many less precise, simple measurements of the zenith sky brightness using Sky Quality Meters closer to the cities and other major sources of light pollution. The Globe at Night program involves citizen scientists from the general public in this increasingly useful, world-wide monitoring effort—as explained at the program website.



  • Info

Cinzano, P., Falchi, F., & Elvidge, C.D., 2001. The first world atlas of the artificial night sky brightness. Monthly Notices of the Royal Astronomical Society 328, 689–707.

Cinzano, P., Falchi, F., 2012. The propagation of light pollution in the atmosphere. Monthly Notices of the Royal Astronomical Society 427, 3337–3357

Krisciunas, K., Semler, D.R., Richards, J., Schwarz, H.E., Suntzeff, N.B., Vera, S., & Sanhueza, P., 2007. Optical Sky Brightness at Cerro Tololo Inter-American Observatory from 1992 to 2006. Publications of the Astronomical Society of the Pacific, 119, 687–696.

Krisciunas, K., Bogglio, H., Sanhueza, P., & Smith, M.G., 2010. Light pollution at high zenith angles, as measured at Cerro Tololo Inter-American Observatory. Publications of the Astronomical Society of the Pacific, 122, 373–377.

Lavasseur-Regourd, A.C. & Dumont, R., 1980. Absolute photometry of zodiacal light. Astronomy and Astrophysics, 84, 277–279.

Marín, C. & Jafar, J. 2008. Starlight: a Common Heritage. UNESCO/IAU/Instituto de Astrofísica de Canarias.

Patat, F., 2008. The dancing sky: 6 years of night-sky observations at Cerro Paranal. Astronomy and Astrophysics, 481, 575–591

Addere Ltda, 2012. Estudio Capacidades y Oportunidades para la Industria y Academia en las Actividades Relacionadas o Derivadas de la Astronomía y los Grandes Observatorios Astronómicos en Chile. Informe No. 3.

Centro de Estudios Avanzados en Zonas Arídas, 2013. Pionera actividad busca potenciar el desarrollo turístico como promotor del conocimiento científico en el Parque Nacional Fray Jorge.

Ministerio del Medio Ambiente, 2013. Establece norma de emissión para la regulación de la contaminación lumínica, elaborada a partir de la revision del decreto No. 686, de 1998, del Ministerio de Economía, Fomento y Reconstrucción. Biblioteca del Congreso Nacional de Chile – Legislación chilena.  Decreto 43. Fecha Publicación:  3 May 2013.  Inicio Vigencia: 3 May 2014.


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Thematic essay: ‘Windows to the universe’: Starlight, dark-sky areas and observatory sites

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