def get_feh(cls, filename):
        """
        example filename: yapsi_w_X0p602357_Z0p027643.dat
        """
        X,Y,Z = cls._get_XYZ(filename)

        Xsun = 0.703812
        Zsun = 0.016188

        return np.log10((Z/X) / (Zsun/Xsun))
example filename: yapsi_w_X0p602357_Z0p027643.dat