Suppose you are looking for sites for a visible-light observatory, an infrared observatory, and a radio observatory. What are the main criteria of excellence for each? What sites are actually considered the best today?

Visible-Light Observatory: The main criteria for a visible-light observatory are clear skies (low cloud cover), high altitude (to reduce atmospheric distortion), low light pollution, and stable weather conditions. The best sites today include the Mauna Kea Observatories in Hawaii, the Paranal Observatory in Chile, and the Roque de los Muchachos Observatory in Spain.

Infrared Observatory: Infrared observatories need high altitudes and dry conditions because water vapor absorbs infrared radiation. They also benefit from cool temperatures to reduce heat noise. The best sites today include the Atacama Desert in Chile (home to the Paranal Observatory and ALMA), the South Pole (home to the South Pole Telescope), and high-altitude balloons or space-based observatories like the Spitzer Space Telescope.

Radio Observatory: Radio observatories need to be in locations with low radio frequency interference, which often means being far from populated areas. They also benefit from being in valleys or other geographic features that shield them from stray signals. The best sites today include the Very Large Array in New Mexico, the Arecibo Observatory in Puerto Rico (though it recently collapsed), and the FAST telescope in China.