page_title: Extremely Large Telescope
section_title: Comparison
hierarchical_section_title: Extremely Large Telescope / Comparison
caption_attribution_description: English: After years of thorough site testing, Cerro Armazones is the chosen site for the construction of the European Extremely Large Telescope (E-ELT). In the top of this mountain, a gigantic dome, Present and future observatories compared to a marvel from the past. The E-ELT dome will be 100 metres in diameter, about the size of the Colosseum in Rome, but atop a 3000-metre mountaintop. For comparison, each of the four VLT unit telescopes are 25 metres high, about the size of an eight-storey building. This picture is based on a preliminary design for the E-ELT.
context_page_description: The Extremely Large Telescope is an astronomical observatory currently under construction. When completed, it is planned to be the world's largest optical/near-infrared extremely large telescope. Part of the European Southern Observatory agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile. The design consists of a reflecting telescope with a 39.3-metre-diameter segmented primary mirror and a 4.2 m diameter secondary mirror, and will be supported by adaptive optics, eight laser guide star units and multiple large science instruments. The observatory aims to gather 100 million times more light than the human eye, 13 times more light than the largest optical telescopes existing in 2014, and be able to correct for atmospheric distortion. It has around 256 times the light gathering area of the Hubble Space Telescope and, according to the ELT's specifications, would provide images 16 times sharper than those from Hubble. The project was originally called the European Extremely Large Telescope, but the name was shortened in 2017.
context_section_description: One of the largest optical telescopes operating today is the Gran Telescopio Canarias, with a 10.4 m aperture and a light-collecting area of 74 m². Other planned extremely large telescopes include the 25 m/368 m² Giant Magellan Telescope and 30 m/655 m² Thirty Meter Telescope, which are also targeting the beginning of the 2020 decade for completion. These other two telescopes roughly belong to the same next generation of optical ground-based telescopes. Each design is much larger than previous telescopes. Even with the reduction in size to 39.3 m, the ELT is significantly larger than both other planned extremely large telescopes. It has the aim of observing the Universe in greater detail than the Hubble Space Telescope by taking images 15 times sharper, although it is designed to be complementary to space telescopes, which typically have very limited observing time available. The ELT's 4.2-metre secondary mirror is the same size as the primary mirror on the William Herschel Telescope, the second largest optical telescope in Europe.
  The ELT under ideal conditions has an angular resolution of 0.005 arcsecond which corresponds to separating two light sources 1 AU apart from 200 pc distance. At 0.03 arcseconds, the contrast is expected to be 10⁸, sufficient to search for exoplanets. The unaided human eye has an angular resolution of 1 arcminute which corresponds to separating two light sources 30 cm apart from 1 km distance.
caption: ELT compared to the VLT and the Colosseum
exif: {"Image ImageDescription": "Present and future observatories compared to a marvel from the past. The E-ELT dome will be 100 metres in diameter, about the size of the Colosseum in Rome, but atop a 3000-metre mountaintop. For comparison, each of the four VLT unit telescopes are 25 metres high, about the size of an eight-storey building. This picture is based on a preliminary design for the E-ELT."}