  High-multiplex and deep spectroscopic follow-up of upcoming panoramic
deep-imaging surveys like LSST, Euclid, and WFIRST is a widely recognized and
increasingly urgent necessity. No current or planned facility at a U.S.
observatory meets the sensitivity, multiplex, and rapid-response time needed to
exploit these future datasets. FOBOS, the Fiber-Optic Broadband Optical
Spectrograph, is a near-term fiber-based facility that addresses these
spectroscopic needs by optimizing depth over area and exploiting the aperture
advantage of the existing 10m Keck II Telescope. The result is an instrument
with a uniquely blue-sensitive wavelength range (0.31-1.0 um) at R~3500,
high-multiplex (1800 fibers), and a factor 1.7 greater survey speed and
order-of-magnitude greater sampling density than Subaru's Prime Focus
Spectrograph (PFS). In the era of panoramic deep imaging, FOBOS will excel at
building the deep, spectroscopic reference data sets needed to interpret vast
imaging data. At the same time, its flexible focal plane, including a mode with
25 deployable integral-field units (IFUs) across a 20 arcmin diameter field,
enables an expansive range of scientific investigations. Its key programmatic
areas include (1) nested stellar-parameter training sets that enable studies of
the Milky Way and M31 halo sub-structure, as well as local group dwarf
galaxies, (2) a comprehensive picture of galaxy formation thanks to detailed
mapping of the baryonic environment at z~2 and statistical linking of evolving
populations to the present day, and (3) dramatic enhancements in cosmological
constraints via precise photometric redshifts and determined redshift
distributions. In combination with Keck I instrumentation, FOBOS also provides
instant access to medium-resolution spectroscopy for transient sources with
full coverage from the UV to the K-band.
