Remove regions of magnitude-magnitude space where the unmasked solid angle is
        statistically insufficient to estimate the background.

        INPUTS:
            solid_angle[1]: minimum solid angle (deg^2)

def _pruneMMD(self, minimum_solid_angle):
        """
        Remove regions of magnitude-magnitude space where the unmasked solid angle is
        statistically insufficient to estimate the background.

        INPUTS:
            solid_angle[1]: minimum solid angle (deg^2)
        """

        logger.info('Pruning mask based on minimum solid angle of %.2f deg^2'%(minimum_solid_angle))

        solid_angle_mmd = self.solid_angle_mmd*(self.solid_angle_mmd > minimum_solid_angle)
        if solid_angle_mmd.sum() == 0:
            msg = "Pruned mask contains no solid angle."
            logger.error(msg)
            raise Exception(msg)

        self.solid_angle_mmd = solid_angle_mmd

        # Compute which magnitudes the clipping correspond to
        index_mag_1, index_mag_2 = np.nonzero(self.solid_angle_mmd)
        self.mag_1_clip = self.roi.bins_mag[1:][np.max(index_mag_1)]
        self.mag_2_clip = self.roi.bins_mag[1:][np.max(index_mag_2)]

        logger.info('Clipping mask 1 at %.2f mag'%(self.mag_1_clip) )
        logger.info('Clipping mask 2 at %.2f mag'%(self.mag_2_clip) )
        self.mask_1.mask_roi_sparse = np.clip(self.mask_1.mask_roi_sparse, 0., self.mag_1_clip)
        self.mask_2.mask_roi_sparse = np.clip(self.mask_2.mask_roi_sparse, 0., self.mag_2_clip)