def create_empty(cls, tstart, tstop, fill=0.0, nside=64):
        """Create an empty livetime cube."""
        cth_edges = np.linspace(0, 1.0, 41)
        domega = utils.edge_to_width(cth_edges) * 2.0 * np.pi
        hpx = HPX(nside, True, 'CEL', ebins=cth_edges)
        data = np.ones((len(cth_edges) - 1, hpx.npix)) * fill
        return cls(data, hpx, cth_edges, tstart=tstart, tstop=tstop)
Create an empty livetime cube.