code: def _ray_step(self, x, y, alpha_x, alpha_y, delta_T):
          
          x_ = x + alpha_x * delta_T
          y_ = y + alpha_y * delta_T
          return x_, y_
docs: ray propagation with small angle approximation
  
          :param x: co-moving x-position
          :param y: co-moving y-position
          :param alpha_x: deflection angle in x-direction at (x, y)
          :param alpha_y: deflection angle in y-direction at (x, y)
          :param delta_T: transversal angular diameter distance to the next step
          :return: