SPECTRAL_CLEAN FFT

          [BUNDLES\]SPECTRAL_CLEAN FFT

    Clean the Fourier Transform along the spectral axis of the  DIRTY  cube,
    and  transform  back  the  Clean  components to retrieve the per-channel
    Clean components. The final Clean image is produced using UV_RESTORE.

    The usual Clean convergence parameters apply, although  CLEAN_NKEEP  may
    need small adjustment because of the non-positive nature of the signal.

    The  Fourier  Transform  leads  to a Complex valued data set, whose Her-
    miticity is not considered. Real and Imaginary parts are  cleaned  sepa-
    rately, so the method is about twice slower than a standard Clean.