A recovered friend: The afocal system

The basic properties of a simple afocal optical system, namely a two-lens Keplerian telescope, are reviewed and compared with measurements that can be easily performed with instrumentation available in a university optical laboratory. The object-image conjugation, along with the Lagrangian invariant of the systems, are made clear by the comparison between theory and experiments, highlighting also the limits of the Gaussian approximation in the chosen experimental realization.