docs: def apcor(self):
          """
          return the aperture correction of for the CCD assocated with the reading.
          @return: Apcor
          """
          if self._apcor is None:
              try:
                  self._apcor = Downloader().download_apcor(self.reading.get_apcor_uri())
              except:
                  self._apcor = ApcorData.from_string("5 15 99.99 99.99")
          return self._apcor