The Pierre Auger Observatory uses 27 large-aperture wide-angle Schmidt telescopes to measure
the longitudinal profile of air showers using the air-fluorescence technique. Up to the year 2013,
the absolute calibration of the telescopes was performed by mounting a uniform large-diameter
light source on each of the telescopes and illuminating the entire aperture with a known photon
flux. Due to the high amount of work and person-power required, this procedure was only carried
out roughly once every three years, and a relative calibration was performed every night to track
short-term changes. Since 2013, only the relative calibration has been performed. In this paper,
we present a novel tool for the absolute end-to-end calibration of the fluorescence detectors, the
XY-Scanner. The XY-Scanner uses a portable integrating sphere as a light source, which has been
absolutely calibrated. This light source is installed onto a motorized rail system and moved across
the aperture of each telescope. We mimic the illumination of the entire aperture by flashing the
light source at ∼1700 positions evenly distributed across the telescope aperture. For the absolute
calibration of the light source, we built a dedicated setup that uses a NIST-calibrated photodiode
to measure the average photon flux and a PMT to track the pulse-to-pulse stability. We present
the laboratory setups used to study the characteristics of the employed light sources and discuss
the inter-calibration between selected telescopes.