There are only so many mountains where you can put a telescope on this Earth and when you’re trying to build the biggest telescope ever you want to choose a good one. That much is obvious, but the selection of a site for a major observatory is no simple matter. Rather, it is the result of a complex interplay of astronomy, geography and economics. If you want to see where the pristine but unfeeling grandeur of the cosmos comes cheek to jowl with the messy subjectivity of human affairs then eavesdrop on a telescope site-selection meeting.Last week, the Thirty Meter Telescope (TMT) consortium announced the result of a long deliberation over where to park their 1,430-ton spyglass, to be built over the next 10 years. This may not seem like a big surprise — after all, Mauna Kea is already home to the largest concentration of major telescopes in the world. But there is a strong contender in Chile named Cerro Armazones that could have come out ahead.
I’m interested in understanding why Hawaii prevailed. Sure enough, there’s a politics-behind-the-science story here. But there’s an even more fascinating science-behind-the-politics story too.
My guide in teasing out the finer points of the TMT decision is Rene Racine, a TMT board member and professor emeritus at the Université de Montréal. Racine has first hand experience with both Hawaiian and Chilean skies, having helped to build one observatory (Univeristy of Toronto Southern Observatory) on Las Campanas and directed another (Canada-France-Hawaii Telescope) on Mauna Kea.
To get an idea of why this decision matters so much let’s consider a few facts. First, TMT is one of a new generation of telescopes that is expected to vault ground-based astronomy to previously unimagined heights. As its name suggests, its primary mirror is 30 meters across. It’s composed of 492 hexagonal segments which collectively will have a light gathering area 144 times that of the Hubble Space Telescope and nearly 10 times the resolution at infrared wavelengths. That’s according to TMT’s own PR, but clearly there’s nothing like it around today.
TMT will also cost between 1 and 2 billion dollars when all is said and done. This is not quite at the scale of the world’s biggest science projects, like the Large Hadron Collider or the James Webb Space Telescope, but it’s getting there. In fact, TMT and other proposed observatories of this generation may end up being the biggest telescopes on Earth for all time because the funding required to go even larger would more logically be directed towards putting telescopes in orbit.
TMT has its roots at Caltech, which operates the mighty Keck telescopes on Mauna Kea. Given the infrastructure at hand on the Hawaiian peak it would seem inevitable that TMT would end up there. Furthermore, for reasons of politics and access, US partners in the project are naturally interested in seeing TMT built on US soil. Canada, another partner, is also well ensconced at Mauna Kea. Ditto for the Japanese, who are looking to buy into TMT. Japan already has its big Subaru telescope on Mauna Kea, so the Japanese like the idea of keeping their eggs in the Hawaiian basket.
But back in 2002, the Office of Hawaiian Affairs filed suit to prevent Keck from adding two small outrigger telescopes citing lack of an adequate environmental assessment. It was a shocking wake-up call for astronomers, who suddenly realized that not everyone with a stake in Mauna Kea was thrilled with the prospect of yet more construction on a site that is sacred to native Hawaiians.
For TMT that means being open to possibilities beyond Mauna Kea, and most of those possibilities are in Chile. Cerro Armazones is a particularly appealing choice. It is within sight of Paranal, which is home to Europe’s most advanced observatory (the VLT) and it may be slightly better for observing. The Chilean government rolled out the red carpet for TMT, giving it the first crack at Armazones.
On top of that, TMT has two competing megascopes that are also on the drawing board. One of those, the 27-meter Giant Magellan Telescope (GMT) has already settled on a location in Chile. The other, the European Extremely Large Telescope (EELT) is giving Chile strong consideration.
(Click here for S&T.com's list of the world's largest optical telescopes.)
That’s the politics. But here’s where the science comes in. From a big telescope’s perspective, the atmosphere has two layers. The lower atmosphere is basically the first few hundred meters or so. It’s the boundary layer where proximity to the surface affects the movement of air. The upper atmosphere is everything else.
Turbulence in the atmosphere is what limits how well optical telescopes can see the sky. It’s like looking at pebbles at the bottom of a stream. The more waves there are on the surface of the water the more distorted the image of the pebbles becomes.
If you’re talking about natural seeing conditions — literally, how sharp the stars appear — then Armazones is close if not better than Mauna Kea despite being a full kilometer lower in altitude. On top of that, Armazones is located in the Atacama Desert, the driest place on Earth. That means it has many more clear nights than Mauna Kea. Hour for hour, any telescope on Armazones is bound to be more productive.
But now let’s bring adaptive optics into the picture. This is the technology that allows a big telescope to monitor air movement and deform its own optics to compensate for atmospheric turbulence. It’s only effective at infrared wavelengths but what it does is amazing. Adaptive optics essentially gives the telescope a view that is similar to what the pebbles in the stream would look like if the water were perfectly still.
Adaptive optics is a big part of TMT’s design. It will work both on Mauna Kea and Armazones, but astronomers expect it will work better on Mauna Kea. This is because the upper atmosphere — the part above the boundary layer — is somewhat less turbulent above Mauna Kea than it is above Armazones. Why? According to Racine it’s partly a function of latitude. Because Mauna Kea is nearer the equator it’s relatively unaffected by the jet streams that flow at higher latitudes both north and south. Armazones’ upper atmosphere is a bit more turbulent in comparison and so somewhat harder for adaptive optics to deal with.
Racine estimates that more than half of TMT’s observing projects will be in the infrared. The likely targets at these wavelengths include the earliest galaxies, the birthplaces of stars and newborn exoplanets — all hot topics. The chance to maximize the impact of adaptive optics on these objects is a big factor in the TMT decision to go with Mauna Kea.
So what about the Office of Hawaiian Affairs? Racine says the dispute over the Keck outriggers is still cause for concern, but the TMT group has made Hawaiian community input a high priority. There’s still the possibility that there will be future problems but in this case the benefits outweigh the risk.
That makes Cerro Armazones the most eligible mountain without an observatory—a fact that will not have escaped the attention of the European Extremely Large Telescope. The Europeans have political factors of their own to deal with, including some strong incentives to locate on La Palma, one of the Spanish Canary Islands. They’ll make their site decision by the end of this year, which means we’ll soon have another chance to see the geopolitics of astronomy in action.
Ivan Semeniuk is host of the podcast The Universe in Mind and a science journalist in residence at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto.