The study of resolved stellar populations of the Milky Way and other Local Group galaxies can provide us with a fossil record of their chemo-dynamical and star-formation histories over many gigayear timescales. The Milky Way is the only galaxy whose components can all be resolved into individual stars. As such it is an ideal, and indeed, unique laboratory to investigate the details of the processes behind formation and evolution of a disc galaxy. As a result, the need to obtain a large-scale empirical description of the Milky Way has defined the ambitious requirements of large photometric and spectroscopic surveys during the last decade.
The Gaia mission is now providing the community with high-precision positional measurements of about one billion stars, with dedicated ground-based surveys (Gaia-ESO, 4MOST, WEAVE, GALAH) in place or planned to provide the spectroscopy for chemistry and kinematics. MOONS will observe up to 1,000 targets across a 25 arcmin field-of-view in the optical and near-infrared (0.6-1.8 um) simultaneously. A high-resolution (R~20,000) setting in the H-band has been designed for accurate determination of stellar abundances such as alpha, light, iron-peak, neutron-capture elements. Therefore, MOONS will provide this crucial follow-up for the Gaia mission and for Galactic surveys with the VISTA telescope (as well as the southern regions observed by Pan-STARRS and UKIDSS), delivering accurate radial velocities, metallicities and chemical abundances for several millions of stars over its lifetime.
MOONS is designed to reach a H∼17.5 mag source (magnitude of the most reddened red-clump stars in the bulge) with a signal-to-noise ratio larger than 60 in less than 4 hrs on-source. The also near-IR APOGEE-II survey, located in the South and thus having a considerably improved observability of the Bulge with respect to APOGEE, will reach a limiting magnitude of H~14, but at a very low signal-to-noise level. Therefore, these two projects are naturally complementary, with MOONS being able to provide a deeper, more complete characterisation across a wide grid of highly-obscured in-plane observations.