By William Cerny, Winter 2020.

With price tags in the hundreds of millions to billions of dollars, modern telescope projects are no small investments. Accordingly, only the observatories that can both optimize the quality of science and overcome a gambit of logistical and political hurdles ultimately come to fruition. The early 2020s promises to revolutionize ground-based astronomy with the next generation of extremely large telescopes, including the 24.5 meter Giant Magellan Telescope (GMT), the 8.4 meter Vera Rubin Observatory (VRO), and the eponymously-sized Thirty Meter Telescope, but even these prominent projects have been challenged over their locations for a myriad of reasons. With such powerful technology being constructed, years of planning and consideration goes into deciding just where these telescopes are ultimately built – so how exactly are observatory sites chosen?

Ground-based telescopes must overcome a variety of natural factors in order to serve as effective scientific instruments. As a first criterion, the site must be suitable for the type of research being conducted. The exact type of site that is desirable depends heavily on the wavelength of light that the observatory hopes to study: some wavelengths of light, including x-ray and gamma ray, fail to penetrate Earth’s atmosphere, and thus can only be studied from orbit. While accessible at Earth’s surface, longer wavelengths such as visual, infrared, and radio waves each suffer distortion from the atmosphere and are subject to challenges from Earth’s climate patterns. Therefore, because high data quality is paramount to high-precision astronomical science, the selection of natural sites that minimize these detrimental effects is of great importance. 

For observations at optical wavelengths, the distortion of incoming light by the atmosphere poses the most significant challenge. Ever since the invention of telescopes in the 17th century, physicists and astronomers have known that the resolution of their optical systems was limited by the atmosphere. Newton notably recognized that the most distant sources of visible light – stars – did not appear as point sources when viewed by eye through a telescope, instead producing “one broad lucid Point” which he believed was composed of “many trembling Points confusedly and insensibly mixed with one another,” an effect he attributed to atmospheric turbulence in all but name. [1] The cause of this atmospheric turbulence, at the most basic level, is the fact that the atmosphere is an inhomogeneous medium. Because of thermal energy from the Sun and human sources, the air is heated unevenly, and is thus full of turbulent air rising and falling in convection currents. Hotter air expands and causes a local decrease in pressure, compared to equilibrium atmospheric pressure, and likewise cooler air causes a local increase in pressure. This change in pressure causes air to vary in index of refraction over time, causing light to take very slightly different paths through the Earth’s atmosphere, leading to point-like stars becoming blurred images. [2] Because orbital observatories such as the Hubble Space Telescope do not look through the Earth’s atmosphere, they are limited only by diffraction due to their mirrors, and not by this blurring. However, space-based telescopes are ultimately limited in their light-gathering power due to the difficulty of sending them to space with large mirrors, rendering them inefficient for very deep looks at the faint Universe. Therefore, ground-based telescopes remain a vital component of modern astronomy. 

In order for optical ground-based astronomical observatories to remain effective, they must seek to minimize atmospheric distortion through careful site selection. One of the key factors to consider is elevation; at higher elevations, there is less atmosphere to look through, and thus incoming light rays suffer less from the refraction described above. At high altitudes, the air is often less turbulent and less dense, leading to smaller convection patterns and a lower index of refraction for air, all of which contributes to reduced blurring. Additionally, extremely dry environments are ideal – in addition to reducing distortion due to water vapor for infrared and submillimeter wavelength observatories, dry environments typically feature little to no cloud cover, increasing the number of nights where observing is impossible. While these may seem like simple constraints to work within, selecting sites with favorable seeing conditions is highly complex and nuanced. An illustrative example of this complexity can be noted by observing that most modern major observatories are on the west coasts of continents. This fact is no coincidence – observatories placed on coastal mountains benefit greatly from the fact that common weather patterns (which typically move east to west) traverse a broad span of open ocean, which reduces the amount of pollutants in the air that passes near the telescopes. Due to the coastal location of these observatory sites, this also means that the air that reaches land does not significantly warm before reaching the observatory, leading to significantly reduced turbulence and near laminar (smooth) flow of air. [3] Conveniently, these locations are often highly isolated from human-induced light pollution, allowing for extremely dark skies year-round. 

Only a very limited number of sites around the world meet all the above conditions – in fact, these conditions are so rare that most major telescopes lie in just two major geographic areas: Mauna Kea, in Hawai’i, and the mountains of the Atacama Desert in Chile. These two locations complement each other, as each site’s telescopes are ultimately limited by their latitude, which determines the area of sky that each cover the Northern Hemisphere and Southern Hemisphere skies, respectively. To put these sites’ significance in perspective, some estimate that nearly 70% of the world’s astronomical infrastructure is located in Chile alone – a number which is only expected to grow with the continuing advent of new, ambitious projects including GMT and VRO/LSST. [4] While other locations, including the American Southwest, the Canary Islands, and Australia still host research-grade telescopes, interest in further developing these locations remains diminished to the desirability of Hawaiian and Chilean sites. 

Despite the advantages of these scientifically optimal locations, constructing observatories in these environments can often prove a significant challenge politically. In Chile and Hawaii, now home to many of the world’s best extremely large class telescopes, the construction of observatories has involved blasting and leveling the peaks of mountains with explosives. The construction also requires the development of significant mountaintop road and building infrastructure – these are necessary for construction and maintenance of the observatory. The development of this level of infrastructure is often controversial, as many question the environmental and wildlife impact of building on previously untouched land. Illustratively, the embattled Large Binocular Telescope incited significant controversy over its positioning in an Arizona forest home to a species of endangered American red squirrels, resulting in a year-long legal battle that ultimately resulted in significant concessions on behalf of the telescope’s construction. [5] An ongoing controversy also surrounds the development of the Thirty Meter Telescope at Mauna Kea, a mountain considered to be sacred by some among Hawaii’s indigenous population. Given the significant cost and longevity of observatory projects, these sociopolitical challenges are taken seriously, and often prompt prolonged legal battles spanning years. The future of astronomical infrastructure will increasingly depend on balancing these interests with the quality of sites in pursuit of astronomy’s most ambitious research aims. 

[1] ​Rowlands, Peter. ​Newton and Modern Physics.​ New Jersey: World Scientific, 2018.
[2] ​Hecht, Eugene. ​Optics.​ Harlow: Pearson Education Limited, 2017.
[3] ​“Atmospheric Effects.” Australia National Telescope Facility. CSIRO Australia Telescope National Facility, May 8, 2019.
[4] ​“Chile to Host 70 Pct of Global Astronomical Infrastructure by 2020.” EFE, April 15, 2015. https://www.efe.com/efe/english/technology/chile-to-host-70-pct-of-global-astronomical-infrastr ucture-by-2020/50000267-2587084.
[5] “Mt. Graham Red Squirrel Refugium.” LBTO Science and Operations. http://abell.as.arizona.edu/~lbtsci/redsquirrel.html.