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June 2, 2023


All about GTC

Once upon a time...

When it comes into operation, the Gran Telescopio CANARIAS (GTC), with a mirror equivalent in size to a 10.4 m diameter circular mirror, will be among the most advanced and best equipped telescopes for astronomical research.

The idea was first put forward in 1987 at the Instituto de Astrofísica de Canarias (IAC), following the inauguration of the Anglo-Dutch William Herschel Telescope (WHT). At that time, Spain and the UK felt it would be appropriate to examine the possibility of building a large telescope at the Observatorio del Roque de los Muchachos (ORM), a powerful new-technology instrument to drive the research of astrophysicists and keep up the momentum of their excellent work to date.

In 1989 they made a joint proposal to the CCI - International Scientific Committee- for a telescope with a monolithic 8 m primary mirror, very similar to other projects starting up in the late 1980s, to be built by Spain and the United Kingdom. In the end, in 1990, the British authorities decided to join the US GEMINI Project and pulled out of the initiative in the Canaries. The IAC was undeterred – it would go it alone.

Why the decision to push ahead before securing partners from other countries?

There is no doubt that the GTC meets the strategic needs of Spanish astronomy. Astrophysics has enjoyed spectacular growth in a short time in Spain, markedly increasing both the number of scientists and the quality and quantity of publications, and prompting this initiative to meet the needs and demands of Spanish astrophysicists.

Equally important, there was no Spanish involvement in the new large telescope projects already under way, such as Keck, GEMINI and the VLT. Spain needed its own state-of-the-art instrument, one that would enhance the ORM and keep it on a par with the world’s best observatories. Convinced that this strategy was vital for the future of Spanish astronomy and technology, the IAC continued to promote the GTC and to seek international participation. 

At the beginning, few people thought that Spain would be able to take such a big project forward alone. International participation was thought essential, as the absence of international partners could well determine whether the telescope could be built at all. For the IAC’s directorate, though, it was clear that once the project was launched other countries and institutions would want to be involved, and that has indeed been the case.  

In 1993, representatives of the Spanish Government’s science policy and the Canarian Government lobbied for European funding so that preliminary studies for the project could begin. 

In 1994 the Canarian Government set up the public sector company Gran Telescopio de Canarias S.A. to conduct a feasibility study for the construction of the GTC at the ORM. The Spanish astronomical community (SEA, Sociedad Española de Astronomía, and all the other Spanish astronomy and astrophysics groups) were consulted and confirmed that they was in favour of the proposal to build a large telescope in the Canaries.

A year later, experts on large telescopes from all over the world met in Tenerife to assess the telescope proposal. This group of experts was asked to consider the options of a monolithic or segmented primary mirror, and they agreed to look at the alternatives in depth. After seeking the advice of experts who had helped build the Keck telescopes in Hawaii, which have segmented primary mirrors, the experts opted for a segmented 10 m primary mirror in place of a monolithic 8 m one.

In 1996 approval was granted for a share issue in GRANTECAN so that the Spanish Government could become a stakeholder, giving the GTC an assurance of 100% Spanish finance (50% from the Spanish Government and 50% from the Canarian Autonomous Community) should international backing not be forthcoming.


The telescope is currently being built. The bulk (70%) of the contracts were awarded to Spanish companies, although a large number of foreign firms are working as subcontractors. The National Institute for Astrophysics, Optics and Electronics (Instituto Nacional de Astrofísica, Óptica y Electrónica, INAOE), and the Institute of Astronomy of the National Autonomous University of Mexico (Instituto de Astronomía de la Universidad Nacional Autónoma de México, IA-UNAM), both financed by Mexico’s National Council of Science and Technology (Consejo Nacional de Ciencia y Tecnología de México, CONACYT), signed agreements on 31 July 2001 committing them to contributions worth 5% of the total cost of building the telescope plus start-up costs. In return they gain 5% of the GTC’s observing time. The two Mexican institutions will also pay 5% of the GTC’s running costs and will exchange observing time on the Large Millimetre Telescope (LMT), a 50 m telescope currently being built by the INAOE and the University of Massachusetts, with the GTC.

At the same time, to develop and consolidate science and technology sharing between the Mexican institutions and the IAC, an Agreement on Cooperation in Astrophysics was signed with the INAOE and the IA-UNAM that provides for exchange programmes for scientists and technicians, as well as collaboration on future instrumentation projects.

The University of Florida Research Foundation, which also showed strong interest in partnership in the project, signed participation agreements on 17 October 2001. These involve 5% financial participation in return for 5% of observing time.

With these agreements the GTC project acquired an international standing.