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Cerro Tololo Interamerican Observatory (CTIO) is an astrophysical observatory located in the Chilean Andes, northeast of La Serena. It is run by the National Optical Astronomical Observatory of the United States, with cooperation from the Chilean government. It is the United States' primary site for conducting ground-based optical astronomy in the southern hemisphere; located at a latitude of -30° south, it provides access to skies at high southerly declinations not visible in the continental United States.


Modern, scientific astronomy in South America dates back to the nineteenth century, with the founding of the Cordoba Observatory in Argentina in 1871. Since then other observatories were founded around the continent, not only by their national governments and universities, but also by astronomers from other parts of the world, eager to have a site from which to access the southern skies. Major organizations like Harvard University and the Carnegie Institution established South American observing programs as early as the end of the nineteenth century, often to complement their existing observatories in South Africa.

The Cerro Tololo observatory was originally a joint project between the Ford Foundation and the National Science Foundation, meant to expand access to southern skies by United States' astronomers, and to establish scientific ties with the Chilean government. The CTIO project began with site surveys, seeing measurements, and photometry in 1963, and by 1966, plans were already in motion to develop Cerro Tololo as a major astrophysical observatory. The official, governmental announcement was made during a joint 1967 press conference by Presidents Eduardo Frei of Chile and Lyndon Johnson of the United States. Several other observatories were developed during this time, including the European Southern Observatory facilities at La Silla, and the Carnegie Institution's observatory at Las Campanas.

The largest telescope on Cerro Tololo is the Blanco 4-meter telescope, used for wide-field imaging and spectroscopy. It was commissioned in 1974, and is a twin of the 4-meter Mayall Telescope on Kitt Peak in southern Arizona. Cerro Tololo has several other telescopes, including

  • a 1.5-meter reflecting telescope,
  • a 0.9-meter reflecting telescope,
  • a 16-inch (0.4-meter) telescope (nicknamed El Enano -- the dwarf),
  • the YALO and 2MASS telescopes controlled by separate international university consortia.

In addition to the telescopes on Cerro Tololo itself, CTIO also hosts the SOAR (Southern Observatory of Astrophysical Research) 4.2-meter telescope, and the Gemini South 8-meter telescope, both on nearby Cerro Pachon. Gemini South is one half of a pair of large telescopes, with the other located in the northern hemisphere on the island of Hawaii.

Cerro Tololo is an excellent place to observe the southern sky, particularly the Galactic bulge which passes directly overhead at night in the austral winter (June-July). Aside from the few buildings and roads, it is still nearly pristine wilderness, so one may see wild foxes and tarantulas on occasion. Unfortunately, like many observatories around the world, it is slowly becoming affected by light pollution, principally from the rapidly growing beach resort at La Serena, only 50 miles away.

A trip to CTIO is a bit like a rite of passage for young professional astronomers in the United States. Often, it is our first view of the southern sky, featuring such sights as the Large and Small Magellanic Clouds, the Coal Sack, the Southern Cross, and the glorious Milky Way. It is of course possible to view parts of the Milky Way from the United States (including the bulge). But at CTIO the grandeur and immensity of our galaxy is immediately apparent. I was completely overwhelmed when I stepped outside in the dark for the first time, seeing an unbroken band of white, a sublime arc from horizon to horizon. Standing on a windswept mountain in a far corner of the world, I was profoundly changed by the immensity of the cosmos on that night.

On a more mundane level, my trip to Chile was also the first time I tried some South American delicacies, including heart of palm and the famous pisco sour. While having one's view of the universe expanded is a good thing, the little things count for something, too. While no alcohol is served on the mountain (you'll have to visit La Serena for the pisco sours), the cafeteria on Tololo is very good, and they make brown-bag "midnight lunches" to order to keep you going overnight. They can also do vegetarian meals without much difficulty, but it helps to know a bit of Spanish if you need to order something.

CTIO, like most other working observatories, is accessible mainly by scientists with reservations; unlike Kitt Peak in Arizona, the observatory itself does not have a visitor center. In principle, any US citizen may observe there by submitting a research proposal to NOAO and providing proof of funding (it costs about US$1500 for an observing run, including airfare, and site housing and support fees).

Other than my own experience, I gathered some information from:
http://www.ctio.noao.edu
Sky and Telescope, v.35, n.2, p.72-6 (1968)
Sky and Telescope, v.32, n.6, p.335-40 (1966)

Revised May 27, 2002.

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