docs: def get_zeropoint(expnum, ccd, prefix=None, version='p'):
      """Get the zeropoint for this exposure using the zeropoint.used file created during source planting..
  
      This command expects that there is a file called #######p##.zeropoint.used which contains the zeropoint.
  
      @param expnum: exposure to get zeropoint of
      @param ccd: which ccd (extension - 1) to get zp
      @param prefix: possible string prefixed to expsoure number.
      @param version:  one of [spo] as in #######p##
      @return: zeropoint
      """
      uri = get_uri(expnum, ccd, version, ext='zeropoint.used', prefix=prefix)
      try:
          return zmag[uri]
      except:
          pass
  
      try:
          zmag[uri] = float(open_vos_or_local(uri).read())
          return zmag[uri]
      except:
          pass
  
      zmag[uri] = 0.0
      return zmag[uri]