  We report on high resolution spectra of the bright QSO HS0747+4259 (zem =
1.90, V = 15.8) observed to search for intermediate redshift OVI absorption
systems. The spectra were obtained by means of the Space Telescope Imaging
Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution
Echelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVI
systems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs =
1.46-1.8 exhibit a sufficient number of lines of different ionic transitions to
estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10
Ryd. All recovered UV ionizing spectra are characterized by the enhanced
intensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau
(1996). This is in line with the observational evidence of a deficiency of
strong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UV
background shows significant local variations: the spectral shape estimated at
z = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause of
these variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to the
line of sight. No features favoring the input of stellar radiation to the
ionizing background are detected, limiting the escape fraction of the galactic
UV photons to f_esc < 0.05.
