  We describe the design and construction of a formatted fiber field-unit,
SparsePak, and characterize its optical and astrometric performance. This array
is optimized for spectroscopy of low-surface brightness, extended sources in
the visible and near-infrared. SparsePak contains 82, 4.7" fibers subtending an
area of 72"x71" in the telescope focal plane, and feeds the WIYN Bench
spectrograph. Together, these instruments are capable of achieving spectral
resolutions of lambda/dlambda ~ 20000 and an area--solid-angle product of ~140
arcsec^2 m^2 per fiber. Laboratory measurements of SparsePak lead to several
important conclusions on the design of fiber termination and cable curvature to
minimize focal ratio degradation. SparsePak itself has throughput >80% redwards
of 5200 A, and 90-92% in the red. Fed at f/6.3, the cable delivers an output
90% encircled energy at nearly f/5.2. This has implications for performance
gains if the WIYN Bench Spectrograph had a faster collimator. Our approach to
integral-field spectroscopy yields an instrument which is simple and
inexpensive to build, yet yields the highest area--solid-angle product per
spectrum of any system in existence. An Appendix details the fabrication
process in sufficient detail for others to repeat. SparsePak was funded by the
National Science Foundation and the University of Wisconsin-Madison Graduate
School, and is now publicly available on the WIYN Telescope through the
National Optical Astronomical Observatories.
