def V(self, brightest=False):
        """
        http://www.aerith.net/astro/color_conversion.html
        """
        mags = self.get_photometry(brightest=brightest, convert=False)
        VT, dVT = mags['VT']
        BT, dBT = mags['BT']
        if (-0.25 < BT - VT < 2.0):
            (a, b, c, d) = (0.00097, 0.1334, 0.05486, 0.01998)
            V = (VT + a - b * (BT - VT) + c * (BT - VT)**2 -
                d * (BT - VT)**3)

            dVdVT = 1 + b - 2*c*(BT-VT) + 3*d*(BT-VT)**2
            dVdBT = -b + 2*c*(BT-VT) - 3*d*(BT-VT)**2
            dV = np.sqrt((dVdVT**2 * dVT**2) + (dVdBT**2*dBT**2))

        else:
            raise ValueError('BT-VT outside of range to convert')

        return V, dV
http://www.aerith.net/astro/color_conversion.html