  In scattering theory the far field pattern describes the directional
dependence of a time-harmonic wave scattered by an obstacle or inhomogeneous
medium, when observed sufficiently far away from these objects. Considering
plane wave excitations, the far field pattern can be written as a function of
two variables, namely the direction of propagation of the incident plane wave
and the observation direction, and it is well-known to be separately real
analytic with respect to each of them. We show that the far field pattern is in
fact a jointly real analytic function of these two variables.
