docs: 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)