Atmospheric conditions at the summit
The summit of Mauna Kea is one of the best places on Earth for ground-based astronomy. Its high elevation at 4,207 meters (13,803 feet) places it above a significant portion of the planet's atmosphere, including most of its water vapor. This extremely dry air is important for observing in infrared and submillimeter wavelengths, which are largely absorbed by atmospheric moisture at lower altitudes. The stable, calm air above the mountain also results in exceptional "seeing," or low atmospheric turbulence, allowing for incredibly sharp images. The Canada-France-Hawaiʻi Telescope (CFHT) site, for example, reports a median seeing of just 0.43 arcseconds.
This unique environment has attracted an international collection of astronomical facilities. The twin W. M. Keck Observatory telescopes are among the largest optical and infrared telescopes in the world, each with a primary mirror 10 meters in diameter. Japan's Subaru Telescope has an 8.2-meter primary mirror, while the Gemini North Telescope has an 8.1-meter mirror. In total, thirteen telescopes operated by up to eleven different countries are located in the Mauna Kea Science Reserve. These instruments have been significant for major discoveries, including providing evidence for the accelerating expansion of the universe—a finding that earned the 2011 Nobel Prize in Physics. They have also helped identify thousands of exoplanets and the supermassive black hole at the center of our Milky Way galaxy.