The phase space mapping, between an object and image plane, characterizes an optical system within geometrical optics framework. We propose a novel idea to characterize the phase mapping in axially symmetric optical systems for arbitrary object locations not restricted to a specific object plane. The idea is based on decomposing the phase mapping into a set a bivariate equations corresponding to different values of the radial coordinate at an specific object surface (most likely the entrance pupil). These equations are then approximated through bivariate Chebyshev interpolation at Chebyshev nodes, which guarantees uniform convergence. Additionally we propose the use of a new concept (effective object phase space), defined as the set of points of the phase space at the first optical element (typically the entrance pupil) that are effectively mapped onto a set of points included at the image surface. The effective object phase space provides a way to avoid tracing rays that do not reach the image surface, by means of an inclusion test.