  Among all neutrino mixing parameters, the atmospheric neutrino mixing angle
theta_{23} introduces the strongest variation on the flux ratios of ultra high
energy neutrinos. We investigate the potential of these flux ratio measurements
at neutrino telescopes to constrain theta_{23}. We consider astrophysical
neutrinos originating from pion, muon-damped and neutron sources and make a
comparative study of their sensitivity reach to theta_{23}. It is found that
neutron sources are most favorable for testing deviations from maximal
theta_{23}. Using a chi^2 analysis, we show in particular the power of
combining (i) different flux ratios from the same type of source, and also (ii)
combining flux ratios from different astrophysical sources. We include in our
analysis ``impure'' sources, i.e., deviations from the usually assumed initial
(1 : 2 : 0), (0 : 1 : 0) or (1 : 0 : 0) flux compositions.
