  We have analyzed the broad absorption line system of the bright (V=16.5)
high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along
with optical data from the MMT and the Steward Observatory 2.3m telescope.
These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms,
corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object
therefore offers a rare opportunity to study broad absorption lines in the
rest-frame extreme UV. We find that the absorption system is dominated by very
high-ionization species, including O VI, NeVIII, and SiXII. We also identify
apparently saturated broad Lyman-series lines of order Ly-gamma and higher.
There is strong evidence for partial occultation of the QSO emission source,
particularly from the higher-order Lyman lines which indicate a covered
fraction less than 0.2. Overall, the data suggest a correlation between a
larger covered fraction and a higher state of ionization. We suggest that the
different covered fractions can be explained by either a special line of sight
through a disk-like geometry or by the existence of density fluctuations of a
factor >2 in the BAL gas. Our photoionization models of the system indicate a
large column density and high ionization state similar to that found in X-ray
``warm absorbers''.
