Full spatial summation reduction. Top panel: comparison of Neptune’s visible spectrum before (black), after adjusting for wavelength-dependent centroid shifts (blue), and after the complete targeted spatial extraction (pink). Bottom panels: residuals (Δ). The blue residual shows the difference between the original spectrum and after the centroid shifts (original - centroid shifted). The pink residuals are the difference between the centroid-shifted spectrum and the final version, thus highlighting the stepwise changes (Centroid shifted-final S/N optimized).