Absolute value of the correlator along a single line of sight as a function of the source redshift z and of the length of the measured spectrum Δ z, for convergence maps reconstructed from Planck (left panel, k _C = 0.021   h Mpc ⁻¹) and ACTPOL (right panel, k _C = 0.064   h Mpc ⁻¹). The value of the resolution of the QSO spectrum is the one predicted for SDSS-III, k _L = 1.9  h Mpc ⁻¹. Also, we set k _l = 0.01  h Mpc ⁻¹. To make the physics of structure formation apparent, we turn off the IGM physics by setting A = β = 1 (it is straightforward to rescale the values of the correlator to reflect different values of A and β).