A Brazilian foundation has pledged $40 million for a giant telescope in Chile — which is good news for astronomers in Tucson.

The mirrors of the Giant Magellan Telescope are being cast and polished at the Steward Observatory Mirror Lab.

Scientists at the University of Arizona astronomy department are members of an international consortium that is building what may be the world’s largest telescope when it begins operation — scheduled for 2021.

Wendy Freedman, chair of the GMT’s board of directors, announced Tuesday that the São Paulo Research Foundation had approved $40 million to assume a partner role in the project.

Costs will be shared by the Ministry of Science and Technology of Brazil, the announcement said.

The Brazilian state of São Paulo joins institutional partners from Australia and Korea, in addition to the UA, The Carnegie Institution for Science, Harvard University, the Smithsonian Institution, Texas A&M University, the University of Chicago and the University of Texas at Austin.

The UA’s Steward Observatory has already cast three of the seven 8.4-meter (27½ feet) mirrors for the telescope.

One has been polished and two are in the queue at the mirror lab beneath Arizona Stadium.

When completed, the Giant Magellan Telescope will combine the light-gathering capacity of seven mirrors into the equivalent of a telescope with a mirror 24½ meters (80 feet) in diameter.

A pad for the observatory building has been cleared at Las Campanas Observatory, a peak in the high, dry Atacama Desert in Chile.

It will be the largest telescope in the world, if it is completed before two other projects currently in development.

The Thirty Meter Telescope, being led by a the University of California, CalTech and a consortium of Canadian universities, will be built on Mauna Kea in Hawaii.

The 39-meter European Extremely Large Telescope is being developed by the European Southern Observatory for placement on Cerro Amazones, also in Chile.

The GMT is farther along than those projects, said Director Patrick McCarthy.

McCarthy said the São Paulo announcement is “the critical element allowing us to go forward with construction.”

McCarthy said the involvement of a robust astronomy community in Brazil adds to the project’s promise.

McCarthy said he hopes to begin construction of a building to house the telescope next year and have it operating by 2021, with at least four of the seven mirrors in place.

Buell Jannuzi, director of Steward Observatory and head of the UA astronomy department, said the GMT board, of which he is a member, expects to make that construction decision within the next couple of months.

The UA continues to produce mirrors, with three cast, one polished and a fourth scheduled to be cast early next year.

The GMT’s eight mirrors (one is a spare) represent $150 million to $170 million in contracts for the mirror lab, Jannuzi said.

The two U.S.-based giant telescopes originally competed for money from the National Science Foundation for construction and operation, but the NSF has no plans to fund either telescope in the next decade.

Both projects now say they will build with foundation, institutional and international involvement. “The U.S. federal budget is oversubscribed — too complex and too slow,” McCarthy said.

McCarthy said he expects the NSF to sign on with the projects at a later date.

That would ensure that astronomers not affiliated with the partner institutions get time on the new, giant scopes, which promise resolution of objects 10 times greater than that achieved by the Hubble Space Telescope.

Contact reporter Tom Beal at tbeal@tucson.com or 573-4158.