  We use Moser's normal forms to study chaotic motion in two-degree hamiltonian
systems near a saddle point. Besides being convergent, they provide a suitable
description of the cylindrical topology of the chaotic flow in that vicinity.
Both aspects combined allowed a precise computation of the homoclinic
interaction of stable and unstable manifolds in the full phase space, rather
than just the Poincar\'e section. The formalism was applied to the
H\'enon-Heiles hamiltonian, producing strong evidence that the region of
convergence of these normal forms extends over that originally established by
Moser.
