def turn_physical_on(self,ro=None,vo=None):
        """
        NAME:
           turn_physical_on
        PURPOSE:
           turn on automatic returning of outputs in physical units
        INPUT:
           ro= reference distance (kpc)
           vo= reference velocity (km/s)
        OUTPUT:
           (none)
        HISTORY:
           2016-01-19 - Written - Bovy (UofT)
        """
        self._roSet= True
        self._voSet= True
        if not ro is None:
            self._ro= ro
        if not vo is None:
            self._vo= vo
        return None