  In this paper we study dynamical properties of the area preserving Henon map,
as a discrete version of open Hamiltonian systems, that can exhibit chaotic
scattering. Exploiting its geometric properties we locate the exit and entry
sets, i.e. regions through which any forward, respectively backward, unbounded
orbit escapes to infinity. In order to get the boundaries of these sets we
prove that the right branch of the unstable manifold of the hyperbolic fixed
point is the graph of a function, which is the uniform limit of a sequence of
functions whose graphs are arcs of the symmetry lines of the Henon map, as a
reversible map.
