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The NPR.org article Hawaii Protesters Block Access Road To Stop Construction Of Massive Telescope briefly summarizes the situation, one paragraph says:

The TMT, as it is called, has been the object of intense opposition for nearly a decade and the building project has faced numerous delays, protests and lawsuits. But a decision by the state's Supreme Court in October cleared the way for construction to begin after reinstating a building permit that was revoked years earlier. And last week, Gov. David Ige announced several roads would be closed beginning Monday to allow for the movement of large equipment onto the mountain.

and another says

"Hawaii is a unique place," Meisenzahl said, noting that protesters gathered on the mountain were engaging in a peaceful, nonviolent form of protest called Kapu Aloha. "It's a small island and everybody knows each other so law enforcement and opponents are working together. Despite what is happening today it's a positive environment," he added.

Question: What would be the scientific impact of locating the Thirty Meter Telescope at its alternate site? Is it a loss of resolution, or infrared spectrum, or both, or something else?

I think the Canary Island site would be the alternate to Mauna Kea, since the E-ELT would be located in Chile and putting both (extremely) large telescopes in the same hemisphere is undesirable.

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The Canary Islands have slightly poorer seeing on average, but the main difference would be the higher column density of water vapour. Mauna Kea is a higher and "drier" site, which means you can observe into the mid-IR.

Looking at the first light instrument package I would guess that WFOS (wide field imaging at 0.3-1 micron) would not be badly affected. But there would be some impact on IRIS (near-IR spectroscopy) towards the long-wavelength end of its range.

The near-IR adaptive optics would also be somewhat affected by the slightly poorer natural seeing, but I'm not sure I can quantify this.

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  • $\begingroup$ And that's just the first light Instruments. Thanks! $\endgroup$ – uhoh Jul 21 '19 at 5:40
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I can't add much to @RobJeffries's answer but here is a blurb from CBS News's Hawaii or Spain? Why experts say location might not matter for world's largest telescope:

"Every once in a while at Mauna Kea, you get one of those magic nights," said University of California, Santa Cruz astronomy and astrophysics professor Michael Bolte, a Thirty Meter Telescope board member. "When the air is super stable above the site, you get images that you simply couldn't get anyplace else."

[...]And most of the same science planned for Hawaii would still get done in Spain — it would just take longer.

"Depending on the kind of science you want to do, it's going to be a 10% hit to a 50% hit in speed," Bolte said. "You are going to have to observe that much longer at La Palma to get the same quality data."

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