Add an entry into the models table for a source based on L-band and
    C-band flux densities.

def add_from_vla_obs (src, Lband, Cband):
    """Add an entry into the models table for a source based on L-band and
    C-band flux densities.

    """
    if src in models:
        raise PKError ('already have a model for ' + src)

    fL = np.log10 (1425)
    fC = np.log10 (4860)

    lL = np.log10 (Lband)
    lC = np.log10 (Cband)

    A = (lL - lC) / (fL - fC)
    B = lL - A * fL

    def fluxdens (freq_mhz):
        return 10. ** (A * np.log10 (freq_mhz) + B)

    def spindex (freq_mhz):
        return A

    models[src] = fluxdens
    spindexes[src] = spindex