def calc_observable_fraction(self,distance_modulus):
        """
        Calculated observable fraction within each pixel of the target region.
        """
        # 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
Calculated observable fraction within each pixel of the target region.