'Comoving transverse distance in Mpc between two redshifts.
This value is the transverse comoving distance at redshift
``z2`` as seen from redshift ``z1`` corresponding to an
angular separation of 1 radian. This is the same as the
comoving distance if omega_k is zero (as in the current
concordance lambda CDM model).
Parameters
z1, z2 : array-like, shape (N,)
Input redshifts.  Must be 1D or scalar.
Returns
d : `~astropy.units.Quantity`
Comoving transverse distance in Mpc between input redshift.
Notes
This quantity is also called the \'proper motion distance\' in
some texts.'

def _comoving_transverse_distance_z1z2(self, z1, z2):

    Ok0 = self._Ok0
    dc = self._comoving_distance_z1z2(z1, z2)
    if (Ok0 == 0):
        return dc
    sqrtOk0 = sqrt(abs(Ok0))
    dh = self._hubble_distance
    if (Ok0 > 0):
        return ((dh / sqrtOk0) * np.sinh(((sqrtOk0 * dc.value) / dh.value)))
    else:
        return ((dh / sqrtOk0) * np.sin(((sqrtOk0 * dc.value) / dh.value)))
