One of the crucial detector systems of the Pierre Auger Observatory is the fluorescence detector
composed of 27 large-aperture wide-angle Schmidt telescopes. In the past, these telescopes
were absolutely calibrated by illuminating the whole aperture with a uniform large-diameter light
source. This absolute calibration was performed roughly once every three years, while a relative
calibration was performed on a nightly basis. In this contribution, a new technique for an absolute
end-to-end calibration of the fluorescence telescopes is presented. For this technique, a portable,
calibrated light source mounted on a rail system is moved across the aperture of each telescope
instead of illuminating the whole aperture at once. A dedicated setup using a combination of
NIST traceable photodiodes to measure the mean intensity and a PMT for pulse-to-pulse stability
tracking has been built for the absolute calibration of the light source. As a result of these
complementary measurements, the pulse-to-pulse light source intensity can be known to the 3.5%
uncertainty level. The analysis of the readout of the PMT camera at each position of the light
source together with the knowledge of the light source emission provides an absolute end-to-end
calibration of the telescope. We will give a brief overview of this novel calibration method and its
current status as well as show preliminary results from the measurement campaigns performed so
far.