def _psi(self, x, y, q, s):
        """
        expression after equation (8) in Keeton&Kochanek 1998

        :param x:
        :param y:
        :param q:
        :param s:
        :return:
        """
        return np.sqrt(q**2 * (s**2 + x**2) + y**2)
def _psi ( self , x , y , q , s ) : return np . sqrt ( q ** 2 * ( s ** 2 + x ** 2 ) + y ** 2 )
expression after equation (8) in Keeton&Kochanek 1998

        :param x:
        :param y:
        :param q:
        :param s:
        :return: