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Item _em_map_eigenvalues.spectrum

Description

    A description of the eigenvalue spectrum for the data set used in the 3d
     reconstruction for the map. A data set consisting of pure noise has a 
     characteristic eigenvalue spectrum which depends on the number of images, 
     the number of image elements and the noise statistics. 
     Since the eigenvalues are only determined by the spacing and number of 
     the sample points, the eigenvalue spectrum is not affected by the signal 
     to noise in the data or the reliability of the orientations. This 
     information is seen from the resolution dependence of the phase residual 
     seen during refinement.
     For the eigenvectors to be significant, the associated eigenvalues should 
     stand out from the noise eigenvalue spectrum.

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