A proposal to build the Thirty Meter Telescope on the former site of another observatory is “feasible,” said a TMT project leader last week.
Last October, Gov. Josh Green and all four of the state’s Congressional delegates signed a letter urging leaders of the TMT project to consider shifting the construction of the controversial observatory to another site on Mauna Kea’s summit.
The letter cited “the Hawai‘i community’s request” to instead build TMT on the site of a decommissioned observatory, rather than disturbing a previously untouched piece of summit land, as the plan has been so far. The letter’s signatories assured that they would work with the Mauna Kea Stewardship and Oversight Authority and the University of Hawai‘i to develop a process for securing the permits necessary for such a shift.
At the MKSOA’s board meeting last Thursday, TMT project manager Fengchuan Liu told the board that the project could still work if it was shifted to the former site of the Caltech Submillimeter Observatory, which was fully dismantled in 2024. The CSO, first activated in 1986, was selected for decommissioning as part of the terms for TMT to receive a permit; the University of Hawai‘i needed to commit to close five existing telescopes by 2033.
“We on the project have taken a preliminary look at this issue from the technical side, scientifically, engineering-wise, and we believe it is feasible to build on the CSO site,” Liu said.
Liu said there are size differences between TMT and CSO: TMT’s proposed footprint, including the observatory dome, support buildings and parking area is roughly five acres, while CSO occupied only about 0.75 acres.
Nonetheless, Liu reiterated that moving the site to the former CSO location could conceivably be done. He suggested that the observatory’s support facility — used to recoat the TMT’s primary mirror segments — could be moved off the mountain to a location at sea level. Liu said this would introduce a logistical burden to the telescope’s operation, but would reduce the observatory’s footprint.
However, the logistics of such a move are still unclear, Liu said. He told the board he does not know whether the process would require new conservation district use permits or new environmental impact studies, and said the TMT’s own board would also need to sign off on the change.
Liu added that the change in site would also introduce minor variances in the observatory’s functionality: he said the CSO location is in a valley lower than most other summit observatories, causing different wind patterns that could impact the telescope’s imaging, although he believes those impacts can be mitigated.
By the same token, Liu said the lower site would also make the facility less visible from around the island than its proposed higher-elevation site.
While there was no public testimony on the matter at last week’s meeting, the board’s discussion last November of Green’s letter drew hundreds of pages of testimony, largely in opposition to the TMT project in general, relocated or otherwise.
One way or another, the future of the project remains unclear. The National Science Foundation announced in 2024 that it could only realistically fund one of two proposed telescopes as part of a “U.S. Extremely Large Telescope” project — either TMT or the Giant Magellan Telescope in Chile.
While the National Science Foundation announced last year that it had removed TMT from consideration — punching a $1.6 billion hole in the observatory’s budget — Congress passed an appropriations bill last month stipulating that the NSF advance both projects into final design review, although with no additional funding granted to either telescope.
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