| Cerro Pachón | |
| Map: | Chile |
| Mapframe: | yes |
| Elevation M: | 2682 |
| Native Name: | --> |
| Country: | Chile |
| Settlement Type: | Use settlement_type= instead of city_type= (deprecated). |
Cerro Pachón is a mountain in central Chile, located east of the city of La Serena in the Coquimbo Region. The mountain is seismically active. It is the site of the 8.1 m Gemini South Telescope of the Gemini Observatory, and the 4.1 m SOAR optical imager. As of 2020, the Vera C. Rubin Observatory is being constructed at this site.
Starting in 1964, Cerro Morado and Cerro Pachón were the subject of routine observations of seeing conditions. Known as astronomical seeing monitors (ASM), these were operated by the Carnegie Institution of Washington.
In 1989, site planning was begun by the National Optical Astronomy Observatory to select two sites for a proposed pair of 8 m telescopes, one in the northern hemisphere and the other in the south. The first telescope would be located on Mauna Kea in Hawaii, while the second would be at the Cerro Tololo Inter-American Observatory (CTIO), possibly on Cerro Pachón. It is the highest peak within the CTIO boundary, rising some 500 m higher than the nearby peaks of Cerro Tololo and Cerro Morado. A site survey showed that the seeing conditions on Cerro Pachón rivalled that on Mauna Kea, and it was chosen as the site for the Gemini South observatory. First light for the newly-commissioned telescope took place on January 18, 2002.
The Andes Lidar Observatory was constructed on Cerro Pachón by the University of Illinois and saw first light in 2009. This is an upper atmosphere facility that is supported by the National Science Foundation. It uses ground-based remote sensing instruments to study the atmosphere, particularly between altitudes of 80 to 110 km.