def get_rn_freqs(core):
    """
    Get red noise frequency array from a core, with error message if noise
    array has not been included.
    """
    if core.rn_freqs is None:
        raise ValueError('Please set red noise frequency array in '
                         ' the core named {0}.'.format(core.label))
    else:
        return core.rn_freqs, core.rn_freqs.size