Effect of the Fourier transform on a 1024 × 1024 vignette of the Sndice image. Left: applied to the original field; right: applied to the 3 PDE transformed fields – ∇_x, ∇_y, Δ²/ ΔxΔy –. The 288 vignettes covering the focal plane are quadratically averaged. The frames are set by |ν_x| and |ν_y| = ν_nyq. The horizontal and vertical lines passing through the center are due to electronic noise. Dashed circles mark radial cuts ν₁ and ν₂ in Fig. 4. The three points β, γ, and δ mark the three real FFT components averaging same name filters (α is at the center).