  The Dark Energy Spectroscopic Instrument (DESI) is a Stage IV ground-based
dark energy experiment that will measure the expansion history of the Universe
using the Baryon Acoustic Oscillation technique. The spectra of 35 million
galaxies and quasars over 14000 square degrees will be measured during the life
of the experiment. We describe the installation of the major elements of the
instrument at the Mayall 4m telescope, completed in late 2019. The previous
prime focus corrector, spider vanes, and upper rings were removed from the
Mayall's Serrurier truss and replaced with the newly-constructed DESI ring,
vanes, cage, hexapod, and optical corrector. The new corrector was optically
aligned with the primary mirror using a laser tracker system. The DESI focal
plane system was integrated to the corrector, with each of its ten
500-fiber-positioner petal segments installed using custom installation
hardware and the laser tracker. Ten DESI spectrographs with 30 cryostats were
installed in a newly assembled clean room in the Large Coude Room. The ten
cables carrying 5000 optical fibers from the positioners in the focal plane
were routed down the telescope through cable wraps at the declination and hour
angle axes, and their integral slitheads were integrated with the ten
spectrographs. The fiber view camera assembly was installed to the Mayall's
primary mirror cell. Servers for the instrument control system replaced
existing computer equipment. The fully integrated instrument has been
commissioned and is ready to start its operations phase.
