Aperture centre placement in a data field (from L+23). Given a data field of area A′ (not necessarily a square field), apertures are only placed within a smaller field A, which lies well within A′. The boundary of A is chosen such that 99.9% of the effective support of Q_θ lies within A (which corresponds to ∼4 θ) if the aperture centre lies on this boundary. To illustrate this, two aperture centres are depicted: ϕ is within A and ϕ″ is outside of A. It can be seen that the circle around ϕ″, illustrating the effective support of Q_θ, extends beyond A′, thus introducing bias into the calculated aperture mass at this point. To exclude this effect, we only average the aperture centre over the red region A.