def gradezero(evtdata):
    """ Only accept counts with GRADE==0.
        
    Parameters
    ----------
    evtdata: FITS data class
        This should be an hdu.data structure from a NuSTAR FITS file.
        
    Returns
    -------

    goodinds: iterable
        Index of evtdata that passes the filtering.
    """

    # Grade filter
    
    grade_filter = ( evtdata['GRADE'] == 0)
    inds = (grade_filter).nonzero()
    goodinds = inds[0]
    
    return goodinds