  Theoretical spectral energy distributions (SEDs) of white dwarfs provide a
powerful tool for cross-calibration and sensitivity control of instruments from
the far infrared to the X-ray energy range. Such SEDs can be calculated from
fully metal-line blanketed NLTE model-atmospheres that are e.g. computed by the
Tuebingen NLTE model-atmosphere package (TMAP) that has arrived at a high level
of sophistication. TMAP was successfully employed for the reliable spectral
analysis of many hot, compact post-AGB stars.
  High-quality stellar spectra obtained over a wide energy range establish a
data base with a large number of spectral lines of many successive ions of
different species. Their analysis allows to determine effective temperatures,
surface gravities, and element abundances of individual (pre-)white dwarfs with
very small error ranges. We present applications of TMAP SEDs for spectral
analyses of hot, compact stars in the parameter range from (pre-) white dwarfs
to neutron stars and demonstrate the improvement of flux calibration using
white-dwarf SEDs that are e.g. available via registered services in the Virtual
Observatory.
