Compute solid angle within the mask annulus (deg^2) as a
        function of mag_1 and mag_2

def _solidAngleMMD(self):
        """
        Compute solid angle within the mask annulus (deg^2) as a
        function of mag_1 and mag_2
        """
        # Take upper corner of the magnitude bin
        mag_2,mag_1 = np.meshgrid(self.roi.bins_mag[1:],self.roi.bins_mag[1:])

        # Havent tested since adding fracdet
        unmasked_mag_1 = (self.mask_1.mask_annulus_sparse[:,np.newaxis]>mag_1[:,np.newaxis])
        unmasked_mag_2 = (self.mask_2.mask_annulus_sparse[:,np.newaxis]>mag_2[:,np.newaxis])
        n_unmasked_pixels = (unmasked_mag_1*unmasked_mag_2*self.frac_annulus_sparse).sum(axis=1)

        self.solid_angle_mmd = self.roi.area_pixel * n_unmasked_pixels

        if self.solid_angle_mmd.sum() == 0:
            msg = "Mask annulus contains no solid angle."
            logger.error(msg)
            raise Exception(msg)