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You should instead choose the noisy bands after the forward PCA. Any and/or all of your spectral bands may contain noise. The PCA or MNF 'pushes' the noise into the higher PCA bands so that often the first few bands contain all of the non-noise information. For example, if I have 10 bands, do forward PCA and find that bands beyond PCA bands 4-5 are very noisy. I may perform the Inverse rotation, using a spectral subset of the first 5 bands, back into image space. The result is still 10 bands but the noise should, in theory, be removed.
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