  The Hobby-Eberly Telescope (HET) Dark Energy Experiment (HETDEX) is
undertaking a blind wide-field low-resolution spectroscopic survey of 540
square degrees of sky to identify and derive redshifts for a million
Lyman-alpha emitting galaxies (LAEs) in the redshift range 1.9 < z < 3.5. The
ultimate goal is to measure the expansion rate of the Universe at this epoch,
to sharply constrain cosmological parameters and thus the nature of dark
energy. A major multi-year wide field upgrade (WFU) of the HET was completed in
2016 that substantially increased the field of view to 22 arcminutes diameter
and the pupil to 10 meters, by replacing the optical corrector, tracker, and
prime focus instrument package and by developing a new telescope control
system. The new, wide-field HET now feeds the Visible Integral-field Replicable
Unit Spectrograph (VIRUS), a new low-resolution integral field spectrograph
(LRS2), and the Habitable Zone Planet Finder (HPF), a precision near-infrared
radial velocity spectrograph. VIRUS consists of 156 identical spectrographs fed
by almost 35,000 fibers in 78 integral field units arrayed at the focus of the
upgraded HET. VIRUS operates in a bandpass of 3500-5500 Angstroms with
resolving power R~800. VIRUS is the first example of large scale replication
applied to instrumentation in optical astronomy to achieve spectroscopic
surveys of very large areas of sky. This paper presents technical details of
the HET WFU and VIRUS, as flowed-down from the HETDEX science requirements,
along with experience from commissioning this major telescope upgrade and the
innovative instrumentation suite for HETDEX.
