Kitt Peak National Observatory, Tuscon, Arizona, USA. With 23 telescopes, this is the largest collection of astronomical instruments in the world. At left is the Mayall 4-metre telescope (completed in 1973), at centre is the Bok 2.3 metre telescope (opened in 1969 and operated by the Steward Observatory, University of Arizona) and at right is the 0.54 metre telescope (operated by the Steward Observatory).