90 research outputs found
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/(\ell+1/2). This extended Limber approximation can be used to test the
accuracy of the Limber approximation and to improve the rate of convergence at
large \ell's. We show that the error in ordinary Limber approximation is
O(1/\ell^2). We also provide a simple expression for the second order
correction to the Limber formula, which improves the accuracy to O(1/\ell^4).
This correction can be especially useful for narrow redshift bins, or samples
with small redshift overlap, for which the zeroth order Limber formula has a
large error. We also point out that using \ell instead of (\ell+1/2), as is
often done in the literature, spoils the accuracy of the approximation to
O(1/\ell).Comment: 7 pages, 6 figure
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