Convergence of PCG solver preconditioned with the two-level a posteriori preconditioner computed for the small-circles scan with K = 128 circles and f_knee = 1.0. Blue lines correspond to the solutions obtained with the deflation subspace operator, Z_D, with different ranks as defined by the number of approximated eigenvectors with eigenvalues smaller than ϵ_tol, as indicated in the legend. Red curve shows the solution with the standard preconditioner, M_BD.