def derivatives(self, x, y, n_sersic, R_sersic, k_eff, center_x=0, center_y=0):
        """
        returns df/dx and df/dy of the function
        """
        x_ = x - center_x
        y_ = y - center_y
        r = np.sqrt(x_**2 + y_**2)
        if isinstance(r, int) or isinstance(r, float):
            r = max(self._s, r)
        else:
            r[r < self._s] = self._s
        alpha = -self.alpha_abs(x, y, n_sersic, R_sersic, k_eff, center_x, center_y)
        f_x = alpha * x_ / r
        f_y = alpha * y_ / r
        return f_x, f_y
returns df/dx and df/dy of the function