  We present the far-UV spectrum of the quasar HS1700+6416 taken with FUSE.
This QSO provides the second line of sight with the HeII absorption resolved
into a Ly alpha forest structure. Since HS1700+6416 is slightly less redshifted
(z=2.72) than HE2347-4342, we only probe the post-reionization phase of HeII,
seen in the evolution of the HeII opacity, which is consistent with a simple
power law. The HeII/HI ratio eta is estimated using a line profile-fitting
procedure and an apparent optical depth approach, respectively. The expected
metal line absorption in the far-UV is taken into account as well as molecular
absorption of galactic H_2. About 27% of the eta values are affected by metal
line absorption. In order to investigate the applicability of the analysis
methods, we create simple artificial spectra based on the statistical
properties of the HI Ly alpha forest. The analysis of the artificial data
demonstrates that the apparent optical depth method as well as the line
profile-fitting procedure lead to confident results for restricted data samples
only (12.0 < log N(HI) < 13.0). The reasons are saturation in the case of the
apparent optical depth and thermal line widths in the case of the profile fits.
Furthermore, applying the methods to the unrestricted data set may mimic a
correlation between eta and the strength of the HI absorption. For the
restricted data samples a scatter of 10 - 15% in eta would be expected even if
the underlying value is constant. The observed scatter is significantly larger
than expected, indicating that the intergalactic radiation background is indeed
fluctuating. In the redshift range 2.58 < z < 2.72, where the data quality is
best, we find eta ~ 100, suggesting a contribution of soft sources like
galaxies to the UV background.
