Ground based telescopes are considered to be the highest priority in observational astronomy due to of their low cost and simple maintenance. Ground based telescopes are usually in chains so as to obtain much clearer and precise images of the dazzling night sky, but this leads to a major problem: the flux of light captured by the chain of telescopes is low. To overcome this problem, scientists have come up with the next generation of ground based telescopes. These telescopes are much larger than the current everyday telescopes used by astronomers, and of them is the Extremely Large Telescope. The Extremely Large Telescope, or ELT, is a ground based observatory THE EXTREMELY TELESCOPE LARGE and the largest optical/near infrared telescope, ”The world’s biggest eye in the sky”. It’s currently under construction and planned to be completed during 2025. Its approximate cost is about 1.5 billion Euros. It is located on top of the Cerro Amazons in the Atacama desert of Northern Chile. It consists of Five mirrors and Five major instrumentation. The ELT is said to have a light gathering area of 256 times than that of the Hubble space telescope and , by its specification, could produce images 16 times sharper than those from Hubble. The ELT uses a novel design with a total of 5 mirrors. The first three mirrors are curved and form a three mirror anastigmat design for excellent image quality. The fourth and fifth mirrors are flat and used in adaptive optics. Three-mirror anastigmat of Paul or Paul-Baker form. A Paul design has a parabolic primary with spherical secondary and tertiary mirrors; A Paul-Baker design modifies the secondary slightly to achieve a flat focal plane.

ELT Mirrors

Primary Mirror

The surface of the primary mirror is about 39.3 metres composing of 798 hexagonal segments, each measuring approximately 1.4m across and 50mm thick.

Secondary Mirror

The secondary mirror is highly convex and aspheric. It is 4.2m in diameter and weighs about 3.5 tonnes. It is cast from Zerodur. It is the largest secondary mirror to ever be employed on a telescope and the largest convex mirror ever produced.

Tertiary Mirror

It is a 3.8m concave mirror also casted from zerodur. In general, a lot of the current telescopes use two curved mirrors to get the image. The ELT uses three curved mirrors to deliver a better final image quality over a larger field of view as compared to a two mirror design telescope.

Quaternary mirror

The 2.4m quaternary mirror is a flat adaptive mirror, and about 2mm thick. It consists of 8000 actuators which readjust the surface at very high time frequencies. The deformable mirror will be the largest adaptive ever made.

ELT DOME

The ELT dome has a height of 74m and a diameter of 86m, making it the largest dome ever built for a telescope. The dome will have a total mass of 5000 tonnes. The observing slit is a single pair of sliding nested doors having a total width of 45.3m. It consists of a windscreen to protect the telescope’s mirrors. The design of the dome provides sufficient ventilation and air conditioning to the telescope. It is equipped with 8 laser guiding units, commonly used in adaptive optics.

INSTRUMENTATION

The ELT is equipped with 5 main instrumentation:

HARMONI

The High Angular Resolution Monolithic and Optical and Near-Infrared Spectrography will function as the telescope’s workhorse instrument and in spectroscopy.

METIS

The Mid-Infrared ELT Imager and Spectrograph will be a midinfrared imager and spectroscopy. Its powerful spectrograph will help astronomers to investigate the basic physical and chemical properties of exoplanets, like their orbital parameters, and structure, temperature, composition, atmosphere, weather, etc. The METIS will also be used in the James Webb space telecope.

MICADO

The Multi Adaptive Optics Imaging Camera for Deep Observation is one of the first light instruments for the ELT and takes adaptive optics technique to the next level. It works with the multiconjugate adaptive optics module (MAORY). It is used in imaging the detailed structure of galaxies at high redshifts, study of individual stars in nearby galaxies and discovering and characterizing of exoplanets using its chronograph mode. It will also be extremely powerful in a high gravitational environment. MICADO also uses single-slit spectrograph which is ideal to obtain spectra of compact objects.

MOSAIC

The multi-object spectrograph is a powerful tool of the ELT which is used to obtain visible and near-infrared spectroscopy of large samples of astronomical objects. It will allow for previously unprecedented observation of objects ranging from stars at the heart of the Milky Way to the distant galaxies at the edge of the observable universe. These future observations will help astronomers in tracing the growth of galaxies, their structure and the distribution of matter from shortly after the big bang to the present day.

HIRES

The High Resolution Spectrograph is a stable spectrograph which is capable of producing high spectral resolution. Its primary goal is to characterise the exoplanetary atmospheres with the ultimate goal of looking for signatures of life. HIRES also allows astronomers to study the evolution of stars and galaxies; identify the signatures of the first generations of stars in the primordial universe and also to determine whether some of the fundamental constants of physics actually change with time or not. Its most exciting goal is to directly measure the expansion of the universe.

THE SCIENCE

Are We Alone?

The ELT’s primary goal is to search for exoplanets. This will include not only the discovery of planets down to earth-masses, but also directing imaging of larger planets and possibly the characterisation of their atmospheres. The ELT’s suite of instruments will help astronomers to probe the earliest stages of formation of planetary systems and to detect water and organic molecules in protoplanetary disks around star systems in the making. This will bring us one step closer in answering the question: Are We Alone?

The First Objects Of The Universe

By looking into the most distant regions of the universe, the ELT will help us to enhance our understanding on the formation of the earliest objects of the universe, such as primordial stars, primordial galaxies and black holes, and the relationship between them. The ELT will also be able to make a detailed study of the evolution of galaxies through cosmic time. Observations of these first galaxies will give us insights toand help us understand how they form and evolve and thereby understanding the pattern of the cosmic web. One of the most important goals is the possibility of making a direct measurement of the acceleration of the universe and also to check possible variations in the fundamental constants with respect to time. A possible ambiguous detection of such variation would have far-reaching consequences for our comprehension of the fundamental laws of physics. Some other science areas include the study of high red-shift galaxies, star formation, the dark age, etc.

The Future

The ELT will help future astronomers to tackle the biggest scientific challenges of our time such as tracking the possibilities of life on other extraterrestrial planets. The ELT is one of the crown jewelsholy grails of modern observational astronomy, which will mark the beginning of the golden era in observational astronomy and bring us one step closer to unlocking the secrets of the Universe”