  High-resolution UV spectra, obtained with HST and FUSE, enable us to analyse
hot hydrogen-rich central stars in detail. Up to now, optical hydrogen and
helium lines have been used to derive temperature and surface gravity. Those
lines, however, are rather insensitive; in particular, neutral helium lines
have completely vanished in the hottest central stars. Therefore, we have
concentrated on ionization balances of metals, which have a rich line spectrum
in the UV, to establish a new temperature scale for our sample. Furthermore, we
have determined abundances of light metals, which had been poorly known before.
They show considerable variation from star to star. We present results of
quantitative spectral analyses performed with non-LTE model atmospheres.
