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The Extremely Large Telescope: All about this huge telescope

The Extremely Large Telescope or the ELT is an astronomical project by ESO or the European Southern Observatory. It is a ground-based observatory that aims to improve our understanding of our universe. The ELT will mainly be looking visible and the near-infrared spectrum of light. After the construction, it will be the largest observatory ever built, even bigger than the James Webb Space Telescope.

An artist’s close-up impression of ELT’s primary mirror. (Image Credit:

The ELT will be based in the Atacama Desert in Chile just like many other ESO’s projects such as the ALMA and the VLT or the Very Large Telescope which is the predecessor of the ELT. This region was selected because of the elevation of the place meaning that there is less atmosphere between the observatory and space and also because the area does not experience cloudy weather which can interfere with the observations of the telescope. These conditions make the region the best place for the ELT to carry out its observations.

The mirrors of the ELT
The ELT has a primary mirror or M1 (short for Mirror 1) that is a whopping and incredible 39.3-metre or 129 feet in diameter. The mirror is segmented into 798 segments to make it easier to clean and replace the mirror. The mirror is in fact so big that it would be able to collect 100 million times more light than the human eye. After the light hits the primary mirror, the light will follow a big path. The primary mirror will reflect the light to the secondary mirror or M2 (short for Mirror 2) which will reflect it to the third mirror or M3 (short for Mirror 3) which will again reflect it to the fourth mirror or M4 (short for Mirror 4) which will again reflect it to the fifth mirror or M5 (short for Mirror 5) which will reflect the light to the instrument or camera. The ELT’s fourth mirror has 8000 actuators which will be used to counter the distortions produced by the Earth’s atmospheric wobble and also from the primary mirror vibrating due to wind. The mirrors and the structure that holds the mirrors altogether weigh around 3700 tonnes!

The Dome
The observatory dome totally weighs around 6100 tonnes, even though many things are done to reduce the weight of the structure! The dome is also protected from earthquakes. To achieve this, the dome was built on massive shock absorbers.

ELT’s structure and the dome (Image Credit:



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I am a young epistemophile aspiring to become a Space Scientist