def derivatives(self, x, y, coeffs, beta, center_x=0, center_y=0):
        """
        returns df/dx and df/dy of the function
        """
        shapelets = self._createShapelet(coeffs)
        r, phi = param_util.cart2polar(x, y, center=np.array([center_x, center_y]))
        alpha1_shapelets, alpha2_shapelets = self._alphaShapelets(shapelets, beta)
        f_x = self._shapeletOutput(r, phi, beta, alpha1_shapelets)
        f_y = self._shapeletOutput(r, phi, beta, alpha2_shapelets)
        return f_x, f_y