Spatially coupled Hessian by the effect of a PSF. We have used a 3 × 3 pixels PSF of FWHM = 2 pixels. Each colored square corresponds to a subspace of n_par × n_par where we have set all elements to one for illustration purposes. In reality the latter would also be scaled according to the products of response functions within each subspace. The relative scaling therefore is only due to the Y term that multiplies each subspace.