  MIRC-X (Michigan InfraRed Combiner-eXeter) is a new highly-sensitive
six-telescope interferometric imager installed at the CHARA Array that provides
an angular resolution equivalent of up to a 330 m diameter baseline telescope
in J and H band wavelengths ($\tfrac{\lambda}{2B}\sim0.6$ milli-arcseconds). We
upgraded the original MIRC (Michigan InfraRed Combiner) instrument to improve
sensitivity and wavelength coverage in two phases. First, a revolutionary
sub-electron noise and fast-frame rate C-RED ONE camera based on a SAPHIRA
detector was installed. Second, a new-generation beam combiner was designed and
commissioned to (i) maximize sensitivity, (ii) extend the wavelength coverage
to J-band, and (iii) enable polarization observations. A low-latency and
fast-frame rate control software enables high-efficiency observations and
fringe tracking for the forthcoming instruments at CHARA Array. Since mid-2017,
MIRC-X has been offered to the community and has demonstrated best-case H-band
sensitivity down to 8.2 correlated magnitude. MIRC-X uses single-mode fibers to
coherently combine light of six telescopes simultaneously with an image-plane
combination scheme and delivers a visibility precision better than 1%, and
closure phase precision better than $1^\circ$. MIRC-X aims at (i) imaging
protoplanetary disks, (ii) detecting exoplanets with precise astrometry, and
(iii) imaging stellar surfaces and star-spots at an unprecedented angular
resolution in the near-infrared. In this paper, we present the instrument
design, installation, operation, and on-sky results, and demonstrate the
imaging and astrometric capability of MIRC-X on the binary system $\iota$ Peg.
The purpose of this paper is to provide a solid reference for studies based on
MIRC-X data and to inspire future instruments in optical interferometry.
