def gen_wavelength_header(h, axis, ax_num = 3):
    """
    Generates a FITS wavelength header in Ansgtroms

    Parameters
    ----------
    h : dict
        The header to update
    axis : 1D :class:`~numpy:numpy.ndarray`
        The regular wavelength axis
    ax_num : int
        (Optional) The axis number to use in the header for the spectral dimmension. Default = 3

    Returns
    -------
    h : dict
        Updated header
    """
    h['NAXIS%d'%ax_num] = len(axis)
    h['CRPIX%d'%ax_num] = 1
    h['CRVAL%d'%ax_num] = axis[0]
    h['CDELT%d'%ax_num] = axis[1]-axis[0]
    h['CUNIT%d'%ax_num] = 'Angstroms'
    h['CTYPE%d'%ax_num] = 'LINEAR'
    return h