code: def calc_observable_fraction(self,distance_modulus):
          
          # This is the observable fraction after magnitude cuts in each 
          # pixel of the ROI.
          observable_fraction = self.isochrone.observableFraction(self.mask,distance_modulus)
          if not observable_fraction.sum() > 0:
              msg = "No observable fraction"
              msg += ("\n"+str(self.source.params))
              logger.error(msg)
              raise ValueError(msg)
          return observable_fraction
docs: Calculated observable fraction within each pixel of the target region.