The DESI instrument

The Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4-m telescope has begun an ambitious survey of spectroscopic measurements of 40 million galaxies and quasars over an area of 14,000 deg2 . DESI is a wide field, multi-object, fiber-fed spectrograph, operating in the wavelength range of 360 - 980 nm. In this paper we present an overview of the DESI instrument focusing on key components including the prime-focus corrector, the focal plane with 5,020 remotely controlled fiber positioners, the procedures to position the fibers on selected targets and the spectrograph system. We then discuss the performance of the instrument during the first year of the DESI survey.