Extended Limber approximation

We develop a systematic derivation for the Limber approximation to the angular cross-power spectrum of two random fields, as a series expansion in (+1/2) ^-1. This extended Limber approximation can be used to test the accuracy of the Limber approximation and to improve the rate of convergence at large 's. We show that the error in ordinary Limber approximation is O (^-2). We also provide a simple expression for the 2nd order correction to the Limber formula, which improves the accuracy to O (^-4). This correction can be especially useful for narrow redshift bins, or samples with small redshift overlap, for which the 0th order Limber formula has a large error. We also point out that using instead of +1/2, as is often done in the literature, spoils the accuracy of the approximation to O (^-1).