Members of the Thirty Meter Telescope (TMT) International Organization have turned down Spain’s €1bn offer to build the instrument at the Roque de los Muchachos Observatory in La Palma. The package was put forward in July with funding also provided from the European Investment Bank and Spain’s Official Credit Institute. The decision makes Hawaii’s Mauna Kea as the only possible option for the telescope.
The TMT, if built, would use a segmented primary mirror consisting of 492 elements of zero-expansion glass for a 30 m-diameter primary mirror that will allow astronomers to resolve the faintest and oldest galaxies.
The TMT’s members include the University of California, the California Institute of Technology, Canada, India, and Japan.
Officials at the TMT have not released a final construction cost for the telescope, but it is expected to be at least $3bn, of which $2bn has been committed.
In 2009, the TMT organization choose Hawaii’s Mauna Kea peak as its location. However, in 2015 protests by indigenous Hawaiians, who regard the site as sacred, delayed the start of construction.
The issue forced officials to consider a change of location with the island of La Palma, belonging to Spain’s Canary Islands, proposed as its preferred alternative site in 2016.
Some astronomers, however, were concerned by the potential move given that La Palma’s environmental conditions may limit the telescope’s scientific potential. In particular, the warmer climate and lower elevation of La Palma compared to Mauna Kea could affect mid-infrared observations, which require dry, cool conditions.
Rival designs
Further woes hit the TMT project beginning in 2024 when the US National Science Foundation (NSF) announced it would only support the construction of the Giant Magellan Telescope (GMT) or the TMT – but not both facilities.
A year later, the NSF decided to pursue the GMT, which uses seven primary and seven secondary mirrors to give it an optical surface of 25.4 m. Building of the GMT is already under way at Chile’s Las Campanas peak with officials working toward a final design review.
In 2025, Spain then offered €400m to join the project if the TMT was built in La Palma. In July, the European Investment Bank and Spain’s Official Credit Institute said it would provide an additional €600m in loans towards construction. Battle for the skies: US insists GMT and TMT telescopes must vie for funding
Yet according to a statement from the Astrophysics Institute of the Canaries (IAC), which operates the Roque de los Muchachos Observatory, TMT officials have now rejected that offer.
The IAC say that TMT members were “unable to reach the consensus required to accept the financial offer”.
The IAC adds that despite the decision, the institute “remains fully committed to the scientific and technological development of the [Roque de los Muchachos Observatory]” and will continue to promote the installation of new infrastructure including the Cherenkov Telescope Array Observatory and the European Solar Telescope.
