Weak Lensing On the Celestial Sphere
  This paper details a description of the pattern of galaxy image distortion over the entire sky caused by the gravitational lensing which is the result of large scale inhomogeneities in our universe. We present a tensor spherical harmonic formalism to describe this pattern, giving many useful formulae. This is applied to density inhomogeneities, where we compute the angular power spectrum of the shear pattern, as well as the noise properties due to finite galaxy sampling and cosmic variance. We show that a detectable level of shear is present for very nearby galaxies, $z\simlt0.2$. For such a shallow sample much of the largest signal-to-noise comes from very large angular scales, $\theta\simgt10^\circ$, although it is in the form of very small shear at a level $\simlt10^{-3}$. 
Albert Stebbins