def air_to_vac(wavelength):
    """
    Implements the air to vacuum wavelength conversion described in eqn 64 and
    65 of Greisen 2006. The error in the index of refraction amounts to 1:10^9,
    which is less than the empirical formula.
    Function slightly altered from specutils.utils.wcs_utils.

    Parameters
    ----------
    wavelength : :obj:'~numpy.ndarray'
        the air wavelength(s) in Angstroms

    Returns
    -------
    wavelength : :obj:'~numpy.ndarray'
        the vacuum wavelength(s) in Angstroms
    """
    #convert wavelength to um from Angstroms
    wlum = wavelength/10000
    #apply the equation from the paper
    return (1+1e-6*(287.6155+1.62887/wlum**2+0.01360/wlum**4)) * wavelength