  A system of 5,020 robotic fiber positioners was installed in 2019 on the
Mayall Telescope, at Kitt Peak National Observatory. The robots automatically
re-target their optical fibers every 10 - 20 minutes, each to a precision of
several microns, with a reconfiguration time less than 2 minutes. Over the next
five years, they will enable the newly-constructed Dark Energy Spectroscopic
Instrument (DESI) to measure the spectra of 35 million galaxies and quasars.
DESI will produce the largest 3D map of the universe to date and measure the
expansion history of the cosmos. In addition to the 5,020 robotic positioners
and optical fibers, DESI's Focal Plane System includes 6 guide cameras, 4
wavefront cameras, 123 fiducial point sources, and a metrology camera mounted
at the primary mirror. The system also includes associated structural, thermal,
and electrical systems. In all, it contains over 675,000 individual parts. We
discuss the design, construction, quality control, and integration of all these
components. We include a summary of the key requirements, the review and
acceptance process, on-sky validations of requirements, and lessons learned for
future multi-object, fiber-fed spectrographs.
