Introducing the Gran Telescopio CANARIAS


The Gran Telescopio CANARIAS (also known as GTC), is an ambitious Spanish project with the aim of constructing and operating one of the largest and most advanced telescopes in the world. With the leadership of the Instituto de Astrofísica de Canarias (IAC), its First Light Ceremony was celebrated in the early morning of 14th July, 2007. Scientific production of the telescope started in March of 2009, once the telescope optics and its control system was sufficiently well developed, and the first science instrument, OSIRIS, had been installed.

twilight picture

The start of the scientific operational phase of the telescope was the culmination of design and development work over more than a decade. The site construction work began during 2000 at the Observatorio del Roque de Los Muchachos, La Palma. This observatory, together with the Observatorio del Teide in Tenerife, form the European Northern Observatory (ENO). The observatory provides excellent observing conditions due to the high quality of the night sky and the existence of a law to protect against light pollution.

In 1994, the public limited company GRANTECAN, S.A. was founded with the aim to design and construct the Gran Telescopio CANARIAS, or GTC. This company was supported by both the Local Government from the Canary Islands and the Spanish Government. GTC also has an international recognition through the agreements signed with the Mexican Government to participate in the project through the Instituto de Astronomía de la Instituto de Astronomía de la Universidad Nacional Autónoma de México y del Instituto Nacional de Astrofísica, Óptica y Electrónica de Puebla. In addition, participation from the United States is through collaboration with the University of Florida.

The ultimate aim of the GTC is to facilitate world-class science observations. Being the largest telescope in the world and thanks to its location at the Roque de los Muchachos Observatory, the telescope will allow the study of key questions in astrophysics such as the nature of black holes, the formation history of stars and galaxies in the early universe, the physics of distant planets around other stars, and the nature of dark matter and dark energy in the universe.

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icon What is the GTC?


The Gran Telescopio CANARIAS (GTC), is currently the largest and one of the most advanced optical and infra-red telescopes in the world. Its primary mirror consists of 36 individual hexagonal segments that together act as a single mirror. The light collecting mirror surface area of GTC is equivalent to that of a telescope with a 10.4m diameter single monolithic mirror. Thanks to its huge collecting area and advanced engineering the GTC classes amongst the best performing telescopes for astronomical research.

telescope picture

The GTC has also a secondary mirror and a tertiary mirror that together with the primary mirror produce the telescope focal plane in the focal station of choice. The scientific instruments that are placed in the focal station then analyse and detect the light, and store the final data.

The telescope mount, a large mechanical, steel structure that holds the mirrors, allows rotational movements of the telescope along a horizontal and vertical axis. This movement has to be extremely precise in order to keep the stars projected stably onto the detector. The telescope is designed so that it is able to observe the optical and infrared light ranges.

GTC will be the last of the generation of so called 8 to 10 meter-class telescopes. Therefore it has tried to improved the design of the predecessors, learning from their experiences.

GTC is the largest telescope thanks to its huge light collecting surface of 75.7 square meters (73 m2 effective area). Apart from its large collecting surface another key feature is the exquisite image quality that the telescope delivers and therefore it can exploit the good sky quality to its maximum. The good image quality is made possible thanks to the active adjustment of the optics. This active optics allows the alignment, deformation and movement the individual segments that form the primary mirror, as well as the alignment of the secondary mirror so as to always keep their optimal position independently of the external conditions (climate, temperature, gravity, manufacture faults, ect...).

In the future GTC will also make use of a technique called "adaptive optics" that will allow the telescope to correct for atmospheric turbulence and hence reach the best possible imaging performance possible, opening up new frontiers for science. For all large telescopes the earth's atmosphere disturbes the light and degrades the image quality drastically, but thanks to adaptive optics this negative effect can be counteracted by applying very fast corrections in real time (some 1000 times per second) making use of a deformable mirror. This is a very demanding technique, but the improvements in image quality and thus for science, are important.

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icon Observing with GTC


The telescope itself is enclosed by a dome that protects it from the elements. At the same time the dome is designed to minimize the turbulence of the air close to the telescope. When an observing night commences than first the dome is opened, allowing the light from the stars to strike the telescope.

control room picture

Control of the telescope and the instruments is done remotely from the control room and highly automated. The available observing time is a valuable commodity. To optimize its use, the observations are scheduled in a way that optimizes the use of the telescope according to the prevailing sky conditions. This so-call queue-schedule observing implies that many programs are simultaneously active and executed according to their scientific priority and taking into account their requirements in terms of observing conditions.

The GTC also makes use of an advanced control system and has a high reliability of operation through a preventive maintenance program designed to locate potential malfunctions before they occur, ensuring that downtime caused by these failures in the system are kept to a minimum.

Part of the preventive maintenance is to ensure that the mirrors remain clean so that they optimally reflect the light. This in practice is not as simple as its sound! The telescope mirrors are regularly cleaned by spraying CO2 snow over its surface. But about every two years each mirror segment comes out of the telescope to put a new reflective aluminium coating on its surface.

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icon GTC in numbers

  • Total telescope moving weight = 400 tonnes
  • Primary mirror weight = 17 tonnes
  • Effective collecting area = 73 m2
  • Effective focal length = 169.9 m
  • Plate scale = 0.82 mm/arcsec
  • Cassegrain focus
    • Instrument weight hanging from rotator = 2400 kg
    • Field-of-view = 15 arcmin in diameter
  • Folded Cassegrain focus
    • Instrument weight hanging from rotator = 1000 kg each
    • Field-of-view = 5 arcmin in diameter
  • Nasmyth focus
    • Instrument weight hanging from rotator = 2400 kg each
    • Instrument weight with a second bearing support = 7500 kg each
    • Field-of-view = 20 arcmin in diameter
Roque de los Muchachos Observatory (Picture courtesy Gianni Tessicini).


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icon Location


The GTC is located at the Roque de los Muchachos Observatory on the island of la Palma. GTC coordinates are: Latitude: 28º 45’ 24’’ N and Longitude: 17º 53’ 31’’ W, at an elevation of about 2300 meters above sea level.

See larger map


The Roque de los Muchachos Observatory (ORM) is one of the prime observatory sites in the world, and without doubt the best site in Europe for ground-based optical and infra-red astronomical observations. It is for that reason GTC was built on this location. The ORM is managed by the Instituto de Astrofísica de Canarias (IAC). More facts about the ORM can be found here.

The ORM is characterized by its overall stable and good climatic conditions that make it favorable for astronomical observations. Being located usually above the inversion layer where clouds tend to form, implies that often the observatory enjoys cloudless skies. But equally important are the low atmospheric turbulence which allows taking advantage of the very good image quality of the telescope with minimum disturbance by the atmosphere.

Large telescopes like the GTC are built in order to study the faintest objects in the sky. Obviously ambient light from cars and street lights hamper the telescopes in their work. Therefore, in order to protect the night skies from light pollution, a sky protection law was established several years ago that has helped ensuring that the ORM will remain a top-quality astronomical observatory for the years to come.

Over the years several telescopes have been constructed at the ORM, and many countries have helped to shape the observatory and its astronomical excellence.

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icon Let's compare


When details on sizes and measures of something are given it is often difficult to understand the dimensions until we convert measures to the human scale. For instance: a forest as large as ten football fields; a distance so large that it would take us a year in a car to get there; a little insect so small that it could rest on the head of a needle. By comparison we can better appreciate the magnitude of things. This is what we now will try to do for the case of GTC.

osiris picture

A telescope as the Gran Telescopio CANARIAS (GTC), given the requirements for proper operation and due to the number of components that form it, can be compared to a lot of things. To get an idea of the vast GTC observation capabilities, we can compare its power of vision to 4 million human eyes and, with it, we could distinguish car headlights some 20,000 km away, or at the distance that separates Spain from Australia. The building of the telescope has a height of 41 m, 6 meters less than the Statue of Liberty in New York. The base of the building that holds the dome must withstand a total weight of 500 tons, comparable to a herd of 62 elephants.

The GTC has a primary mirror of about 10.4 m in diameter, composed of 36 segments of about 450 kg each. In other words, only one of these mirror segments weights the same as a bull. However, despite their weight, the thickness of each segment does not exceed 8 cm. Primary mirrors with smaller overall diameter, such as the one at the Very Large Telescope (VLT), of 8.2 m, located at Cerro Paranal (Chile), can be up to 17.5 cm thick, and hence the GTC mirror is very thin for its size.

One of the most striking peculiarities of the mirrors of the GTC is the dedication with which they have been designed and produced. The limit of the polishing error for the vitro-ceramic material may not exceed 15 nanometers, or 3,000 times smaller than a human hair (a nanometer is a thousandth of a micron, or 0.000001 mm), while any irregularity cannot exceed 90 nm. The polishing of the mirrors achieves this nearly perfect surface. In comparison, if the mirror would be as big a Iberian Peninsula, the highest "mountain" irregularity would be only a few centimeters tall!

The main material of the mirror of the telescope is the Zerodur, equipment similar to that used for the manufacture of the vitro-ceramics. A crucial characteristic of Zerodur is its low coefficient of expansion, which implies that temperature variations in the telescope have no effect on the optics.

Despite the size and weight of the large segmented mirror, all the components must be at a distance from each other of just 3 mm and kept in place with very high tolerance. This is accomplished through a system of sensors and positioners that continuously keep the mirrors in perfect position.

The construction of the GTC has involved numerous pieces and constitutes an important engineering achievement. For instance, the metal structure of the dome has been made of about 59,000 pieces: about 16,000 screws (4,000 kg), some 43,000 nuts (1,500 kg) and about 450 kg of washers.

As a final detail, it is noted that the more than 400 tons of telescope and instrumentation is supported on only a very thin layer of lubricant. The telescope "floats" on hydraulic oil, providing a very smooth movement that requires little force. The telescope may be moved by a slight push of the hand!

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Last modified: 16 December 2020

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