def cxy(self):
        """
        `SExtractor`_'s CXY ellipse parameter in units of pixel**(-2).

        The ellipse is defined as

            .. math::
                cxx (x - \\bar{x})^2 + cxy (x - \\bar{x}) (y - \\bar{y}) +
                cyy (y - \\bar{y})^2 = R^2

        where :math:`R` is a parameter which scales the ellipse (in
        units of the axes lengths).  `SExtractor`_ reports that the
        isophotal limit of a source is well represented by :math:`R
        \\approx 3`.
        """

        return (2. * np.cos(self.orientation) * np.sin(self.orientation) *
                ((1. / self.semimajor_axis_sigma**2) -
                 (1. / self.semiminor_axis_sigma**2)))
`SExtractor`_'s CXY ellipse parameter in units of pixel**(-2).

        The ellipse is defined as

            .. math::
                cxx (x - \\bar{x})^2 + cxy (x - \\bar{x}) (y - \\bar{y}) +
                cyy (y - \\bar{y})^2 = R^2

        where :math:`R` is a parameter which scales the ellipse (in
        units of the axes lengths).  `SExtractor`_ reports that the
        isophotal limit of a source is well represented by :math:`R
        \\approx 3`.