The stability of the fluorescence telescopes of the Pierre Auger Observatory
is monitored with the optical relative calibration setup. Optical fibers
distribute light pulses to three different diffuser groups within the optical
system. The total charge per pulse is measured for each pixel and compared with
reference calibration measurements. This allows monitoring the short and long
term stability with respect of the relative timing between pixels and the
relative gain for each pixel. The designs of the LED calibration unit (LCU) and
of the Xenon flash lamp used for relative calibration, are described and their
capabilities to monitor the stability of the telescope performances are
studied. We report the analysis of relative calibration data recorded during
2004. Fluctuations in the relative calibration constants provide a measure of
the stability of the FD.Comment: 4 pp. To appear in the proceedings of 29th International Cosmic Ray
Conference (ICRC 2005), Pune, India, 3-11 Aug 200