docs: def _get_zeropoint(expnum, ccd, prefix=None, version='p'):
      """
      Retrieve the zeropoint stored in the tags associated with this image.
      @param expnum: Exposure number
      @param ccd: ccd of the exposure
      @param prefix: possible prefix (such as 'fk')
      @param version: which version: p, s, or o ?
      @return: zeropoint
      """
      if prefix is not None:
          DeprecationWarning("Prefix is no longer used here as the 'fk' and 's' have the same zeropoint.")
  
      key = "zeropoint_{:1s}{:02d}".format(version, int(ccd))
      return get_tag(expnum, key)