Giant Magellan Telescope (GMT)

The Giant Magellan Telescope (GMT) is a 25.4-meter telescope under construction at Las Campanas Observatory in Chile’s Atacama Desert. Its primary mirror combines seven of the world’s largest mirrors into 368 square meters of collecting area. The first mirror was cast on November 3, 2005, and the seventh in 2023; site construction started in 2015. About 40% of the project was under construction in 2025, backed by nearly US$1 billion in private funding, the largest private investment in ground-based astronomy. The estimated total cost is about US$2 billion.

Flagship instruments

• Seven giant mirror segments forming one 25.4-meter primary.
• Partners: an international consortium of 15, including Carnegie, the University of Arizona, Harvard and the Smithsonian (see the CfA), KASI in Korea, FAPESP in Brazil, ASIAA in Taiwan and the Weizmann Institute. MIT joined in 2025.

Recent progress

On June 12, 2025, the US National Science Foundation advanced GMT into its Major Facilities Final Design Phase and declined to advance the rival Thirty Meter Telescope. That decision made GMT the single US-backed extremely large telescope. Like the ESO Extremely Large Telescope, it is designed for questions current 8-to-10-meter telescopes cannot answer, such as studying the atmospheres of exoplanets in far more detail than today’s first CO2 detection with JWST allowed.

What to watch, 2026-2030

• 2026-2027 (no fixed date published): completion of the NSF Final Design Phase and Final Design Review, prerequisites for federal construction funding.
• Completion: expected in the 2030s.

Why scientists are converging here

The US extremely-large-telescope program has narrowed to one project, and GMT is it. For American astronomers who want access to a 25-meter-class telescope for astrobiology, galaxies and cosmology, the final design review is the decision to watch.