BABAK TAFRESHI / SCIENCE PHOTO LIBRARY BABAK TAFRESHI / SCIENCE PHOTO LIBRARY
Kitt Peak National Observatory (KPNO) domes, Tucson, Arizona, USA, against stars in a night sky. With 23 telescopes, KPNO is the largest collection of astronomical instruments in the world, with two of them shown here. At right is the Mayall 4-metre telescope (completed in 1973) and at left is the Bok 2.3-metre telescope (opened in 1969 and operated by the Steward Observatory, University of Arizona).
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