The Kitt Peak National Observatory’s (KPNO) telescope, called the Mayall, has been reinvented for a mission to understand the physics of dark energy, an unknown form of energy that is believed to permeate all of space and accelerate the expansion of the universe. To prepare for its new mission, the Mayall will close temporarily over the next 15 months to undergo the largest overhaul in its history in preparation for the installation of the Dark Energy Spectroscopic Instrument (DESI), a massively parallel optical spectrometer capable of measuring the spectra of 5000 astronomical objects simultaneously. The U.S. Department of Energy's Lawrence Berkeley National Laboratory is leading the project's international collaboration. Kitt Peak National Observatory. Courtesy of P. Marenfeld & NOAO/AURA/NSF. When the Mayall first opened its eye to the sky 45 years ago, it was one of the largest optical telescopes in existence. Designed to be versatile, its mission was to assist astronomers in addressing the wide diversity of astronomical questions facing the field. Tremendously successful, it played an important role in many astronomical discoveries, such as establishing the role of dark matter in the universe from measurements of galaxy rotation and determining its scale and structure. "The entire top end of the telescope, which weighs as much as a school bus and houses the telescope's secondary mirror and a large digital camera, will be removed and replaced with DESI instruments," said Lori Allen, KPNO director. With its sturdy construction and ability to view a large swath of sky at a time, the Mayall is the perfect partner for DESI. "This day marks an enormous milestone for us," said Michael Levi, director of DESI. "Now we remove the old equipment and start the yearlong process of putting the new stuff on." When DESI is installed, it will come equipped with a state-of-the-art array of 5000 swiveling robots, each carefully choreographed to point a fiber optic cable at a preprogrammed sequence of galaxies and quasars. The fiber optic cables will carry the light from these objects to 10 spectrographs. The cylindrical, fiber-toting robots will reposition to capture a new spectrum roughly every 20 min. "We started the DESI project by surveying the large telescopes in the world to find one that had a suitable mirror and wouldn't collapse under the huge weight of the instrument," said David Schlegel, Berkeley Lab's DESI project scientist. Over a five-year period, DESI will survey 30 million galaxies and quasars and create the largest map to date of the universe out to a distance of 10 billion light years. The survey will commence in early 2020. More than 465 researchers from about 71 institutions are participating in the DESI collaboration. "We're fired up to get the Mayall ready for its next big adventure,” said David Sprayberry, KPNO site director for DESI.