Illustration of the estimation of the statistic Ξ. The area A′ is the size of the full convergence field, which we convolve with filter functions of scale radius θ, illustrated by the circles. The convolution results at positions α within the smaller area A only depend on κ within A′, while for positions α′ outside of A, information on κ outside of A′ is needed for an unbiased estimate. Therefore, the border of A′ outside of A is discarded before obtaining Ξ.