Angular distances in metric theories
  The general expression of the angular distance between two point sources as measured by an arbitrary observer is given. The modelling presented here is rigorous, covariant and valid in any space-time. The sources of light may be located at a finite distance from the observer. The aberration and the gravitational deflection of light are treated in a unified way. Assuming the gravitational field to be weak, an explicit expansion of the angular separation within the post-post-Minkowskian approximation is carried out. The angular separation within the post-Newtonian approximation truncated at the order $1/c^3$ is straightforwardly derived. 
Pierre Teyssandier (1), Christophe Le Poncin-Lafitte (2) ((1) SYRTE   Observatoire de Paris, (2) Lohrmann Observatory, Dresd...