  MSE is an 11.25m telescope with a 1.5 sq.deg. field of view. It can
simultaneously obtain 3249 spectra at R=3000 from 360-1800nm, and 1083 spectra
at R=40000 in the optical. Absolutely critical to the scientific success of MSE
is to efficiently access the faint Universe. Here, we describe the adopted
systems engineering methodology to ensure MSE meets the challenging sensitivity
requirements, and how these requirements are partitioned across three budgets,
relating to the throughput, noise and fiber injection efficiency. We then
describe how the sensitivity of MSE as a system was estimated at the end of
Conceptual Design Phase, and how this information was used to revisit the
system design in order to meet the sensitivity requirements while maintaining
the overall architectural concept of the Observatory. Finally, we present the
anticipated sensitivity performance of MSE and describe the key science that
these capabilities will enable.
