docs: def zmag(self,):
          """
          Return the photometric zeropoint of the CCD associated with the reading.
          @return: float
          """
          if self._zmag is None:
              hdulist_index = self.get_hdulist_idx(self.reading.get_ccd_num())
              self._zmag = self.hdulist[hdulist_index].header.get('PHOTZP', 0.0)
          return self._zmag