TMT Candidate Sites

TMT selected five sites as candidate observatory sites and began on-site testing in 2003. The site selection process started by considering as complete a list as possible of potentially interesting sites everywhere in the world. It was narrowed down using existing knowledge from previous site selection studies and from established observatory sites. The final list of candidate sites was selected based on satellite studies of cloud cover and precipitable water vapor done by Dr. D.A. Erasmus.

The table below lists the names of the candidate sites, their coordinates and the date ranges during which equipment was set up. Note that data are not necessarily available for the entire date range for each site for a number of reasons (such as equipment failures). See data availability for a table containing the date ranges for each instrument at each site. Click on the names in the table to go to the descriptions of the sites.

Site NameElevation [m]Latitude [deg N]Longitude [deg W]Start TestingEnd Testing
Cerro Tolar 2290 m -21.9639 70.0997October 2003March 2007
Cerro Armazones 3064 m -24.5800 70.1833October 2004November 2009
Cerro Tolonchar 4480 m -23.9333 67.9750 October 2005November 2008
San Pedro Mártir 2830 m 31.0456 115.4691 October 2004August 2008
Mauna Kea 13N 4050 m 19.8330 155.4810 June 2005June 2008

In addition to the testing of the TMT candidate sites, sets of TMT site testing equipment were also installed at Cerro Tololo Inter-American Observatory (CTIO) and at Palomar Observatory for shorter amounts of time for testing and calibration purposes. The data taken during these periods are also available through the database.


Cerro Tolar

A low elevation site (2290 m) in northern Chile, Cerro Tolar is in the Atacama desert and has an extremely arid climate. Tolar is located at a distance of only 8 km from the coast, at 16 km from the closest paved road and 18 km north-north-east of Tocopilla, a town of 25,000 inhabitants. [Note: All distances given here are straight line distances. Driving distance are usually 50-100% longer.] The closest commercial port, airport and major population center is Antofagasta (population 225,000), 190 km south of Tolar. There is a primitive four-wheel drive road to the summit, where some radio equipment is installed.

In spite of its closeness to Tocopilla, light pollution is not an issue, as the bluffs above Tocopilla are ~1000 m high and block most of the light produced in town, but some glow from town is visible nevertheless. Some small light sources from mines are visible in the south and south-east.


Cerro Armazones

Cerro Armazones, a medium elevation site (3064 m) in northern Chile, is also located in the Atacama desert and close to the coast (36 km), with a climate very similar to that of Tolar. It is 22 km from ESO's Very Large Telescope (VLT) on Cerro Paranal and 110 km south of Antofagasta, the closest city. A good, but steep and narrow switch-back road to the summit exists. The closest paved road is ~18 km from Armazones, connected by a rough dirt road. Armazones is the site of a small observatory operated by the Universidad Católica del Norte in Antofagasta. This observatory is not located on the summit, but on a saddle ~350 m below the summit. A new observatory utilizing a hexapod mounted telescope is being commissioned by the University Bochum on Cerro Murphy, a small peak 1.5 km south-west of Armazones and ~225 m lower.

The only lights visible from Armazones are glows close to the horizon from Antofagasta in the north-north-east and from the large, albeit distant Mina Escondida (120 km to the east-north-east). Some prospecting is going on in the area and should be monitored.


Cerro Tolonchar

Cerro Tolonchar is the eastern-most of the Chilean sites, south of the Salar de Atacama, and only 25-80 km from several 5000-6000 m peaks of the Andes. Because of its eastern location and higher altitude, it experiences more precipitation and clouds than Tolar and Armazones, especially during the 'Bolivian Winter' from approximately mid December to mid February. Tolonchar is also the highest (4480 m) and most remote of all TMT candidate sites. The closest settlement is Socaire (300 inhabitants) 38 km to the north, with the closest towns being Toconao (550 inhabitants) at 80 km, San Pedro de Atacama (an eco-tourism town of 1,500 people) at 115 km and Calama, the next large city with a commercial airport (120,000 inhabitants), at 190 km. The driving time is currently 2 h from Socaire, 3 h from San Pedro de Atacama and 4.5 h from Calama. Antofagasta, 250 km distant, can be reached via a different route in ~5.5 h. Currently, only a rough 4WD road exists from the Paso Sico road to the base of Tolonchar (closest distance ~17 km). TMT has constructed a road from the base to the summit

Some lights of mines in and around the Salar de Atacama, approximately 50 km distant but with a direct line of sight, from the Mina Escondida and from the towns described above are visible close to the horizon, but Tolonchar remains a very dark site.

San Pedro Mártir

San Pedro Mártir (SPM) is located in northern Baja California, Mexico, inside a national park and is the site of the Observatorio Astronómico Nacional de San Pedro Mártir. It is a medium-elevation site (2830 m), ~65 km from the Pacific coast in the west and 55 km from the Sea of Cortez (Gulf of California) to the east. The terrain is gently rising from the north, west and south, with a steep cliff dropping more than 2000 m to the desert in the east. The highest point of the area and, in fact, of Baja California, Picacho del Diablo (3095 m), is approximately 6 km to the south-east of the observatory. The area is inside a pine forest and receives more precipitation than the other TMT candidate sites, although most of that comes down in a number of strong events with mostly clear time in between. The closest town is Ensenada (300,000 inhabitants) at 4 h driving time and 140 km line-of-sight distance. The closest commercial airports are in Tijuana (at 220 km) and San Diego (250 km). There is an existing road all the way to the observatory. It is paved to the national park ranger station, ~8 km from the observatory

The surrounding area is very dark from the north-east through the south to the west. In northern directions, the San Diego/Tijuana and Mexicali/Yuma (180/200 km) areas and Ensenada in the north-west produce visible glows, but due to their distance, San Pedro Mártir remains a very dark site.


Mauna Kea 13N

The TMT candidate site on Mauna Kea on the Big Island of Hawaii is a location referred to as '13 North' (13N) on the northern shield, approximately 150 m below the summit. It is adjacent to the Submillimeter Array (SMA) extension area. With ~4050 m elevation, 13N is the second highest of the TMT candidate sites. The conditions are usually dominated by a stable north-easterly flow, but can produce severe weather and precipitation, in particular in the winter.

The lights from most of the towns in the north and west of the Big Island are visible from the 13N location, as well as the glow from Hilo. However, as for all the other sites, the vertical extent of the light pollution remains well below the 65 degree zenith angle observing limit of TMT.