All Figures

Figure 1: Top: comparison of NLTE model atmosphere fluxes ( , ) calculated with different chemical composition. Short dashes: pure H model, thick line: H+He, thin line: H-Ni with [Z] = 0. Note that the main deviations of the blackbody (long dashes, 110 kK) to the synthetic spectra occur shortwards of the indicated H I and He II ground state thresholds. Lower panel: Detail of the upper panel; in addition, a H-Ni model with [Z] = -1 (thin line), and three blackbody fluxes (long dashes, 90, 110, 130 kK) are shown. The prominent He II and O V absorption edges are indicated. Note the drastic decrease of the flux level at shorter wavelengths, if the opacities of metals are considered.

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In the text

	Figure 2: Comparison of NLTE model atmosphere fluxes ( , , solar abundances) without (thick line) and with consideration of iron-group opacities. For clarity, the spectra are convolved with a Gaussian of (the original H-Ni flux is shown in addition, shifted down by 0.5 dex).
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In the text

	Figure 3: Parameters of available NLTE model atmosphere fluxes. The H-Ni models are limited to   because Kurucz's line lists (1996) provide only data up to ionization stage IX.
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In the text

	Figure 4: Comparison of NLTE model atmosphere fluxes ( ) calculated with solar (top) and halo (bottom) abundance ratios at three different temperatures.
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In the text

	Figure 5: Comparison of NLTE model atmosphere fluxes ( , ) calculated with a solar (H-Ni, thin line) and a hydrogen-deficient (He:C:N:O = 33:50:2:15 by mass, thick line) composition. Note the strong differences at energies higher than the indicated H I, He I, and He II ground state thresholds). For clarity, the spectra are convolved with a Gaussian of .
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In the text