The big TMT news last week was followed almost immediately by news of protests regarding the telescope’s location on Maunakea in Hawaii. That’s a complex issue which has prompted lots of interesting discussion, but not something I’m ready to address here. What I thought I would do instead is provide a little background on how decisions are made about where to put telescopes, particularly from a Canadian perspective. Here I’m talking about telescopes that observe visible and near-infrared light (300-2000 nm or so); telescopes which work at other wavelengths have somewhat different requirements.

So, what do you have to think about in determining where to put your giant telescope?

  • Altitude. Not because putting your telescope higher gets it closer to the stars (that tiny difference isn’t important) but because that gets it above some of the Earth’s atmosphere, which absorbs and distorts light.

  • Weather. Visible and near-infrared telescopes can’t see through clouds and precipitation, so a place where it doesn’t rain too much is good. Some wind can be OK, but really strong winds make telescopes shake and their images blur.

  • Seeing. Atmospheric motions make telescope images blurrier (hence why the Hubble Space Telescope has been so successful – no atmosphere!), and the amount of blurring differs from place to place depending on topography and climate.

  • Latitude. Some trade-offs here: telescopes near the poles have longer nights, at least some of the time, but can observe less of the sky over the course of the year. Telescopes near the equator can see the whole sky but can’t get the really long nights needed for some types of observations.

  • Dark skies. Usually this means ‘far away from large cities’ but other activities (eg large mining operations) can also be light pollutors.

  • Accessibility. Getting a telescope somewhere usually requires a road (although there are quite a few telescopes at the South Pole) and a to put the telescope (sometimes you can make your own). Space telescopes are great but mighty expensive to get to!

  • Other environmental factors. Telescopes can be damaged by earthquakes and forest fires so those can be considerations for location. Plus you want your telescope and its construction not to cause damage itself.

  • Human factors. Who owns the land? Does it have spiritual, cultural, or archeological significance? Do the relevant goverments support the project? Are there places for the people who will build and operate the telescope to live? Who will pay for it? These factors are at the heart of the discussion about TMT.

Picking a place for a telescope requires some compromise among all of these factors. Looking at where large telescopes are located now, “tall mountains near oceans and/or deserts” is a pretty common theme: Chile, Hawaii, Mexico, Spain, Arizona, California, the Canary Islands. Those tend to be the places with the best seeing, and seeing is really important since (everything else being equal), better seeing means better data.

In Canada, well, all of our tall-mountains-near-oceans are in pretty wet locations, and we are short of deserts. Back in the 1960s, there was a plan to put a large telescope on Mount Kobau in BC’s Okanagan, which is quite dry. This didn’t happen, and my understanding is that Canadian astronomers of the day decided instead to put their efforts toward joining what became the Canada-France-Hawaii Telescope, CFHT. The sites of older telescopes in Canada, like the David Dunlap Observatory and the Dominion Astrophyscial Observatory were fine when they were built, and priorities of the factors above were different, but are not considered adequate any more. Canadian astronomers don’t use overseas telescopes just so that we can have a holiday in the sun; it really is hard to find good places to put telescopes. More recently there has been some testing of potential telescope sites in Canada’s Arctic although none have yet been developed. It will be interesting to see where this goes in the future.

For more about telescope site selection, this page from NOAO is worth checking out.