code: def V(self, brightest=False):
          
          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
docs: http://www.aerith.net/astro/color_conversion.html