Approximate analytical calculations of photon geodesics in the   Schwarzschild metric
  We develop a method for deriving approximate analytical formulae to integrate photon geodesics in a Schwarzschild spacetime. Based on this, we derive the approximate equations for light bending and propagation delay that have been introduced empirically. We then derive for the first time an approximate analytical equation for the solid angle. We discuss the accuracy and range of applicability of the new equations and present a few simple applications of them to known astrophysical problems. 
Vittorio De Falco, Maurizio Falanga and Luigi Stella
astro-ph.HE