Various methods exist to determine the temperature of the central stars of
planetary nebulae.  The most important of these are the 4 following methods:
1)Zanstra method
2)Energy balance method
3)NLTE study of the central star spectrum
4)Modelling the degree of nebular ionization
The first three methods are known to be dependent on various uncertain
assumptions concerning the central star energy distribution, the nebular
optical depth, and/or the physical state of the central star atmosphere.  They
therefore give conflicting and controversial temperatures.
The fourth method, for which we plan to use ISO observations, does not require
any assumptions regarding either the optical depth or the spectral energy
distribution.  In fact, it is able to determine this latter quantity if a
sufficient range of ionization stages are observed.  The difficulty is that in
the optical and IUE ultraviolet only 'low' ionization stages are seen, the
highest being the difficult to observe NeV state which requires 97 ev to
produce it.  Thus not enough information is available to distinguish
temperatures above 100000 K.  The ISO SWS is able to observe lines of much
higher ionization potential, up to 303 ev.  Using these observations it will
not only be possible to accurately determine high temperatures, but it will be
possible to determine the actual spectral distribution of the central star.