def sparse_counts_map(self):
        """ return a counts map with sparse index scheme
        """
        if self.hpx._ipix is None:
            flatarray = self.data.flattern()
        else:
            flatarray = self.expanded_counts_map()
        nz = flatarray.nonzero()[0]
        data_out = flatarray[nz]
        return (nz, data_out)
return a counts map with sparse index scheme