def get_cdelt(hd):
    """ Return wavelength stepsize keyword from a fits header.

    Parameters
    ----------
    hd: astropy.io.fits header instance

    Returns
    -------
    cdelt: float
      Wavelength stepsize, or None if nothing suitable is found.
    """
    cdelt = None
    if str('CDELT1') in hd:
        cdelt = hd[str('CDELT1')]
    elif str('CD1_1') in hd:
        cdelt = hd[str('CD1_1')]
    return cdelt