Title: Spectral Calibration of the Fluorescence Telescopes of the Pierre Auger Observatory,
Abstract: We present a novel method to measure precisely the relative spectral response
of the fluorescence telescopes of the Pierre Auger Observatory. We used a
portable light source based on a xenon flasher and a monochromator to measure
the relative spectral efficiencies of eight telescopes in steps of 5 nm from
280 nm to 440 nm. Each point in a scan had approximately 2 nm FWHM out of the
monochromator. Different sets of telescopes in the observatory have different
optical components, and the eight telescopes measured represent two each of the
four combinations of components represented in the observatory. We made an
end-to-end measurement of the response from different combinations of optical
components, and the monochromator setup allowed for more precise and complete
measurements than our previous multi-wavelength calibrations. We find an
overall uncertainty in the calibration of the spectral response of most of the
telescopes of 1.5% for all wavelengths; the six oldest telescopes have larger
overall uncertainties of about 2.2%. We also report changes in physics
measureables due to the change in calibration, which are generally small.