code: def _dD_dR(self, R):
          
          HNn_R_sina = self._HNn / R / self._sin_alpha
  
          return HNn_R_sina * (0.3 * (HNn_R_sina + 0.3 * HNn_R_sina**2. + 1) / R / (0.3 * HNn_R_sina + 1)**2
                               - (1/R * (1 + 0.6 * HNn_R_sina) / (0.3 * HNn_R_sina + 1)))
docs: Return numpy array of dD/dR from D1 up to and including Dn.