  The absolute calibration of an air fluorescence detector (FD) is an important
element in correctly determining the energy of detected cosmic rays. The
absolute calibration relates the flux of photons of a given wavelength at the
detector aperture to the electronic signal recorded by the FD data acquisition
system. For the Auger FDs, the primary absolute calibration method uses a
diffusive surface which is placed in front of a telescope aperture to uniformly
illuminate the telescope field of view with a known light signal. This
single-wavelength measurement (375 nm) will be made at intervals of several
months until the stability of the telescopes is determined. The relative
wavelength dependence of the calibration is determined through independent
measurements. The error in absolute calibration at a single wavelength is
estimated to be less than 10%. Two other absolute calibration methods are used
to provide an independent verification of the primary measurement. The
stability of the calibration with time is monitored nightly by a relative
calibration system. In this paper we will provide descriptions of the absolute
and relative calibration methods used by the Auger air fluorescence
observatory. Results from the calibration of the Auger Engineering Array
telescopes will also be presented.
