Analysis geometry used for computing the cross correlation of Zernike coefficients. a_l is the same as a vector of Zernike coeffcients associated to an infinite altitude beacon in the center of the FOV, measured within a virtual aperture with radius ρ_l centered at the origin which is indicated with dashed circle. The solid circle at the bottom is the telescope aperture. (a) Covariance between a_i,j with ; (b) covariance between with ; (c) covariance between a_l,i with a_l,j.