Largest telescope ever sits on a 9,993-foot mountain and is taking 17 years to be built
Authored By Anamica Singh
Published: May 13, 2025, 08:48 IST | Updated: May 13, 2025, 08:48 IST
The European Southern Observatory (ESO) is building the Extremely Large Telescope, which will be the world's largest optical telescope in history. Photos Trending Science & Tech
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Largest telescope ever
The European Southern Observatory (ESO) is building the Extremely Large Telescope, which will be the world's largest optical and near-infrared extremely large telescope in history.
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Being built on 9,993-foot mountaintop in Atacama
It is being built on the 9,993-foot mountaintop in the Atacama Desert in Chile. The giant telescope was planned in 2011, and its construction was initially expected to be over by 2025.
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ELT will see first light 17 years after construction started
Construction on the site started in 2012, and the first stone of the telescope was ceremonially laid in 2017. But it only passed the halfway mark in 202,3 and now the completion and first light are set for March 2029.
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Largest dome ever built for a telescope
The dome of the Extremely Large Telescope is the showstopper. It is 260 ft from the ground and has a diameter of 305 ft. This makes it the largest dome ever built for a telescope. The dome has a total mass of around 6,100 tonnes.
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129 feet wide primary mirror
It has a primary mirror that is 129 feet wide and will consist of a staggering 798 hexagonal segments, all manufactured in Germany. To keep the mirror clean and highly reflective, two segments will be recoated and replaced every day.
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Secondary mirror is challenging
The secondary mirror is posing a challenge since it is highly convex and aspheric. It is 14 ft in diameter and weighs 3492 kg. It is the largest secondary mirror ever used in an optical telescope, and also the largest convex mirror ever produced.
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A tertiary mirror
The 12 ft concave tertiary mirror will be a unique feature in the Extremely Large Telescope, which will divert the light to a convenient focus.
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Largest telescope ever
The European Southern Observatory (ESO) is building the Extremely Large Telescope, which will be the world's largest optical and near-infrared extremely large telescope in history.
9 / 14
Being built on 9,993-foot mountaintop in Atacama
It is being built on the 9,993-foot mountaintop in the Atacama Desert in Chile. The giant telescope was planned in 2011, and its construction was initially expected to be over by 2025.
10 / 14
ELT will see first light 17 years after construction started
Construction on the site started in 2012, and the first stone of the telescope was ceremonially laid in 2017. But it only passed the halfway mark in 202,3 and now the completion and first light are set for March 2029.
11 / 14
Largest dome ever built for a telescope
The dome of the Extremely Large Telescope is the showstopper. It is 260 ft from the ground and has a diameter of 305 ft. This makes it the largest dome ever built for a telescope. The dome has a total mass of around 6,100 tonnes.
12 / 14
129 feet wide primary mirror
It has a primary mirror that is 129 feet wide and will consist of a staggering 798 hexagonal segments, all manufactured in Germany. To keep the mirror clean and highly reflective, two segments will be recoated and replaced every day.
13 / 14
Secondary mirror is challenging
The secondary mirror is posing a challenge since it is highly convex and aspheric. It is 14 ft in diameter and weighs 3492 kg. It is the largest secondary mirror ever used in an optical telescope, and also the largest convex mirror ever produced.
14 / 14
A tertiary mirror
The 12 ft concave tertiary mirror will be a unique feature in the Extremely Large Telescope, which will divert the light to a convenient focus.
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