Improving the accuracy of estimators for the two-point correlation function

Abstract

Aims. We show how to increase the accuracy of estimates of the two-point correlation function without sacrificing efficiency. Methods. We quantify the error of the pair-counts and of the Landy & Szalay estimator by comparing them with exact reference values. The standard method, using random point sets, is compared to geometrically motivated estimators and estimators using quasi-Monte Carlo integration. Results. In the standard method, the error scales proportionally to 1/√Nr, with Nr being the number of random points. In our improved methods, the error scales almost proportionally to 1/Nq, where Nq is the number of points from a low-discrepancy sequence. We study the run times of the new estimator in comparison to those of the standard estimator, keeping the same level of accuracy. For the considered case, we always see a speedup ranging from 50% up to a factor of several thousand. We also discuss how to apply these improved estimators to incompletely sampled galaxy catalogues.