Comparison of three spectral whitening strategies: a: no whitening; b: spectral whitening based on a robust estimate of the covariance computed over a large area (A = 5000 pixels, shown at the bottom); c: spectral whitening based on a local estimate of the covariance computed over a smaller area (A = 300 pixels, shown at the bottom) by masking out regions of high wwS/N given by method (b).