use local reference files as fallback

             'shutter': '/crdsroot/references/msc/csst_msc_ms_shutter_11_000001.fits'}

        """

        if self.crds.is_available:

            try:

                print("CRDS available, use refs from CRDS ...")

                return self.crds.retry(

                    self.crds.get_refs, 3,

                    file_path=self.l0_detector(detector)

                )

        else:

            print("CRDS unavailable, use refs from local files ...")

            except BaseException as e:

                print("CRDS reference access failed! ", e)

        print("Use refs from local files ...")

        refs = dict()

        if self.datatype == "mbi":

            ref_types = ["bias", "dark", "ledflat", "shutter"]