  Wide-field multi-object spectroscopy is a high priority for European
astronomy over the next decade. Most 8-10m telescopes have a small field of
view, making 4-m class telescopes a particularly attractive option for
wide-field instruments. We present a science case and design drivers for a
wide-field multi-object spectrograph (MOS) with integral field units for the
4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to
take advantage of a future prime-focus corrector and atmospheric-dispersion
corrector that will deliver a field of view 2 deg in diameter, with good
throughput from 370 to 1,000 nm. The science programs cluster into three groups
needing three different resolving powers R: (1) high-precision
radial-velocities for Gaia-related Milky Way dynamics, cosmological redshift
surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity
dispersions (R = 10,000) and (3) high-precision stellar element abundances for
Milky Way archaeology (R = 20,000). The multiplex requirements of the different
science cases range from a few hundred to a few thousand, and a range of
fibre-positioner technologies are considered. Several options for the
spectrograph are discussed, building in part on published design studies for
E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data
for exploitation of important imaging surveys planned for the coming decade,
but will also serve as a test-bed to optimize the design of MOS instruments for
the future E-ELT.
