  The measurement of the ultra high energy cosmic ray (UHECR) spectrum is
strongly affected by uncertainties on the reconstructed energy. The
determination of the presence or absence of the GZK cutoff and its position in
the energy spectrum depends not only on high statistics but also on the shape
of the energy error distribution. Here we determine the energy error
distribution for fluorescence telescopes, based on a Monte Carlo simulation.
The HiRes and Auger fluorescence telescopes are simulated in detail. We analyze
the UHECR spectrum convolved with this energy error distribution. We compare
this spectrum with one convolved with a lognormal error distribution as well as
with a Gaussian error distribution. We show that the energy error distribution
for fluorescence detectors can not be represented by these known distributions.
We conclude that the convolved energy spectrum will be smeared but not enough
to affect the GZK cutoff detection. This conclusion stands for both HiRes and
Auger fluorescence telescopes. This result differs from the effect of the
energy error distribution obtained with ground detectors and reinforces the
importance of the fluorescence energy measurement. We also investigate the
effect of possible fluorescence yield measurement errors in the energy
spectrum.

Astrophysics;High Energy Physics - Experiment;High Energy Physics - Phenomenology