@staticmethod
def _ray_step(x, y, alpha_x, alpha_y, delta_T):
    """
        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: transverse angular diameter distance to the next step
        :return: co-moving position at the next step (backwards)
        """
    x_ = x + alpha_x * delta_T
    y_ = y + alpha_y * delta_T
    return x_, y_
