Llano de Chajnantor Observatory is the name for a group of
astronomical
Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include ...
observatories
An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysics, oceanography and volcanology are examples of disciplines for which observatories have been constructed.
Th ...
located at an altitude of over 4,800 m (15,700 ft) in the
Atacama Desert of northern
Chile
Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...
. The site is in the
Antofagasta Region approximately 50 kilometres (31 mi) east of the town of
San Pedro de Atacama. The exceptionally
arid climate of the area is inhospitable to humans, but creates an excellent location for millimeter,
submillimeter
Submillimetre astronomy or submillimeter astronomy (see spelling differences) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation) of the electromagnetic spectrum. Astronomers plac ...
, and mid-infrared astronomy.
This is because
water vapour absorbs and
attenuates submillimetre radiation. Llano de Chajnantor is home to the largest and most expensive astronomical
telescope
A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
project in the world, the
Atacama Large Millimeter Array (ALMA). Llano de Chajnantor and the surrounding area has been designated as the Chajnantor Science Reserve (Spanish: ''Reserva Científica de Chajnantor'') by the government of Chile.
Site description
The Llano de Chajnantor is located on the western side of the
Puna de Atacama, which is another name for the southern part of the
Altiplano. The main ridge of the
Andes
The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the List of longest mountain chains on Earth, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range ...
is over to the east, well into
Argentina
Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
. The
Salar de Atacama
Salar de Atacama, located south of San Pedro de Atacama, is the largest Salt pan (geology), salt flat in Chile. It is surrounded by mountains and lacks drainage outlets. To the east, it is enclosed by the main chain of the Andes, while to the wes ...
basin borders the Puna de Atacama to the west, which in turn is bordered by the
Cordillera Domeyko. The western side of the Puna de Atacama is dotted with the volcanoes of the Central Volcanic Zone of the
Andean Volcanic Belt. The Llano de Chajnantor site itself is bounded by volcanic peaks of the
Purico Complex, which have been active in the
Holocene
The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
but have not erupted in historic times.
Cerro Chajnantor is to the north, Cerro El Chascón to the east, and smaller peaks to the south and west. The Pampa la Bola lies to the northeast, north of Cerro El Chascón and east of Cerro Chajnantor. Llano de Chajnantor has an average elevation of , while Pampa la Bola averages . The thin atmosphere makes work difficult for humans, so much of the activity for ALMA will be conducted at a
base camp
Mountaineering, mountain climbing, or alpinism is a set of outdoor activities that involves ascending mountains. Mountaineering-related activities include traditional outdoor climbing, skiing, and traversing via ferratas that have become sp ...
in the Salar de Atacama basin at approximately in elevation.
Rainfall at the ALMA site averages annually.
The dry climate of Llano de Chajnantor is due to three factors: the
rain shadow
A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side.
Evaporated moisture from body of water, bodies of water (such as oceans and larg ...
s created by the Andes and the
Chilean Coastal Range, the
inversion created by the
Humboldt Current off the coast of Chile, and dry air descending between the
Hadley cell and the
Ferrel cell
Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, but ...
, which forms the
South Pacific High.
While the site is generally viewed as being in the
Atacama Desert, in terms of
ecoregion
An ecoregion (ecological region) is an ecological and geographic area that exists on multiple different levels, defined by type, quality, and quantity of environmental resources. Ecoregions cover relatively large areas of land or water, and c ...
s it is in the
Central Andean dry puna. Llano de Chajnantor is at the same
latitude
In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
as
deserts
A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the l ...
in southern Africa and central Australia.
Telescopes
Telescopes on Llano de Chajnantor

* The
Atacama Large Millimeter Array (ALMA) is a large
submillimetre radio telescope
A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...
interferometer at Llano de Chajnantor, consisting of 54 big and 12 smaller parabolic antennas. Due to the high projected total cost of over $1.3 billion, ALMA has been developed and is run by a consortium led by the
National Radio Astronomy Observatory and the
European Southern Observatory, which are consortiums themselves.
It has been in full operation since October 2013.
* The
Atacama Pathfinder Experiment (APEX) is a
submillimetre radio telescope installed in 2003 and inaugurated in 2005. Its design is based on the North American
ALMA prototype antenna modified for single dish use. It is operated by a consortium led by the
Max Planck Institute for Radio Astronomy.
Telescopes on Pampa la Bola
* The
Atacama Submillimeter Telescope Experiment (ASTE) is a
submillimetre radio telescope built by
Mitsubishi Electric. It was an early prototype for the antennas of ALMA, and is operated by a consortium led by the
National Astronomical Observatory of Japan. It was first built in Japan in 2000, and then was reassembled in Chile in 2002. It began regular observations in 2004.
* The
NANTEN2 Observatory (NANTEN2) is millimeter-wavelength radio telescope operated by a consortium led by the
Nagoya University. In 2004 it was moved from
Las Campanas Observatory, where it had operated since 1995.
Telescopes on adjacent peaks

* The
University of Tokyo Atacama Observatory (TAO) is a optical and infrared telescope under construction (as of 2019) on Cerro Chajnantor, which is immediately north of Llano de Chajnantor. A test facility, the miniTAO, with a telescope was completed in 2009. It is currently the
highest permanent astronomical observatory in the world.
* The
Cerro Chajnantor Atacama Telescope (CCAT) is a proposed
submillimetre radio telescope which will be located on Cerro Chajnantor near the TAO. Originally called the Cornell Caltech Atacama Telescope earlier in the development process, it is now referred to on the project's website by the CCAT acronym. Completion was at one time expected in 2020.
The CCAT project has had trouble finding funding, and construction has not began as of 2019. The scientific collaboration has decided to build a pathfinder facility, CCAT-prime (CCAT-p), before pursuing the full CCAT. CCAT-p will be a similar type of telescope at CCAT, but much smaller, with 6 metre diameter. CCAT-p started construction in 2017 (fabrication started in late 2018) and first light is expected 2021.
* The
Huan Tran Telescope (HTT) is part of a project to measure the polarization of the
cosmic microwave background radiation
The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
.
It is a
Gregorian telescope. Attached to the telescope is the
POLARBEAR experiment, which is an array of
bolometers cooled to less than . HTT was first installed for testing at the
CARMA site in 2010. It was moved to a location on Cerro Toco near the ACT in 2011 and saw
first light in January 2012.
It was developed by a consortium led by the
University of California, Berkeley
The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California), is a Public university, public Land-grant university, land-grant research university in Berkeley, California, United States. Founded in 1868 and named after t ...
.
* The
Simons Observatory is a microwave observatory consisting of a 6-m Large Aperture Telescope (LAT) and three 0.5 m Small Aperture Telescopes (SATs) with a total of 60,000
transition-edge sensor bolometers. It will conduct a wide survey of the
cosmic microwave background
The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
and its linear polarization with arcminute resolution and a sensitivity an order of magnitude higher than the
Planck space telescope. The SATs began observation April 2024, while the LAT's first light is expected early 2025.
Former telescopes
* The
QU Imaging Experiment (QUIET) telescope was a three-element radio telescope array designed to measure the
polarization of the
cosmic microwave background radiation
The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
. The telescopes were custom-designed of an unusual Mizuguchi-Dragone design fitted with highly sensitive
bolometers. The project, led by the
University of Chicago
The University of Chicago (UChicago, Chicago, or UChi) is a Private university, private research university in Chicago, Illinois, United States. Its main campus is in the Hyde Park, Chicago, Hyde Park neighborhood on Chicago's South Side, Chic ...
, was installed in 2009 in the facility that previously housed the CBI array. It operated until 2010 and was dismantled in 2011.
* The
Cosmic Background Imager
The Cosmic Background Imager (or CBI) was a 13-element astronomical interferometer perched at an elevation of 5,080 metres (16,700 feet) at Llano de Chajnantor Observatory in the Chilean Andes. It started operations in 1999 to study the cosmi ...
(CBI) was a radio telescope interferometer designed to measure the intensity and polarization of the
cosmic microwave background radiation
The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
. It operated with thirteen antennas between 1999 and 2006, and from 2006 until 2008 with antennas. The CBI facility was later reused by the QUIET experiment.
* The Millimeter-wave Interferometer (MINT) was a heterogenous four-element array that operated on the slopes of Cerro Toco in late 2001. The prototype instrument contained two and two
Cassegrain reflectors. It was designed to measure the cosmic microwave background radiation.
* The
Mobile Anisotropy Telescope (MAT, or MAT/TOCO) was a telescope that was designed to measure the
anisotropy of the
cosmic microwave background radiation
The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
. It originally was the gondola of the
QMAP balloon-borne experiment. It operated on the slopes of Cerro Toco in late 1997 and again in late 1998.
* The
Atacama Cosmology Telescope (ACT) was a
Gregorian telescope designed to survey the sky at
microwave
Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s. It was installed on the western side of
Cerro Toco in 2007 and was operated by a consortium led by
Princeton University
Princeton University is a private university, private Ivy League research university in Princeton, New Jersey, United States. Founded in 1746 in Elizabeth, New Jersey, Elizabeth as the College of New Jersey, Princeton is the List of Colonial ...
until its decommissioning in 2022.
See also
*
List of astronomical observatories
*
List of highest astronomical observatories
* Other observatories in Chile:
**
Paranal Observatory
***
Very Large Telescope
**
Cerro Armazones Observatory
***
Extremely Large Telescope
**
La Silla Observatory
**
Cerro Tololo Inter-American Observatory
*
Purico Complex, a compound volcano
References
External links
*
Atacama Large Millimeter/sub-millimeter Arraypublic site homepage
Description of a trip to Llano de Chajnantor Observatory on BBC News
{{Portal bar, Chile, Stars, Spaceflight, Solar System, Science
Astronomical observatories in Chile
Atacama Desert
Buildings and structures in Antofagasta Region
European Southern Observatory
1999 establishments in Chile