DESI was mounted on the 4-meter Mayall Telescope at Kitt Peak National Observatory. Kitt Peak National Observatory (KNPO) is located 56 miles southwest of Tucson, Arizona, in the Schuk Toak District on the Tohono O’odham Nation. KPNO is administered by the NSF’s National Optical-Infrared Astronomy Research Laboratory (NSF’s OIR Lab) in Tucson. The Mayall Telescope is a reflector telescope with a 4-meter primary mirror that sits on an equatorial mount. It is the largest of the 22 optical telescopes located on Kitt Peak. It was commissioned in 1973 and it is nearly identical to the 4-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile, also operated by NSF’s OIR Lab and commissioned in 1976.
An extensive rebuilding process began at the Mayall in February 2018 to prepare for and install the Dark Energy Spectroscopic Instrument (DESI). In this transition, the telescope will now have a sole-purpose-use for deep and wide spectroscopic surveys with DESI. The Mayall was chosen to host DESI because of its unique combination of adaptable optical and mechanical designs, and the availability of a large block of dedicated survey time needed to carry out the DESI project.