Within 8 years, the current microlensing surveys of M31 will provide several
hundred events affecting unresolved stars. They will thus allow a statistical
study of the dark matter in M31's halo. The NGST will resolve these stars and
constrain the mass of the corresponding lenses. In case of on-line alerts from
ground-based observations, real-time NGST follow-up with high signal-to-noise
ratio will provide further constraints on the lenses. In addition, high
resolution observations with NGST will complement XMM and the previous optical
data and thus enable a closer insight of X-ray binaries within M31 to be
obtained. The optimal instrumentation to achieve these scientific goals will be
discussed. Last, the study of the dark matter encompassed in the galaxy
clusters would be possible with high angular resolution observations on a large
field camera and would open a new field of research.Comment: 5 pages -- presented at the NGST Science and Technology Exposition
(Hyannis, USA) 13-16 september 1999, published by the PAS
