All Figures

	Figure 1: Model twilight sky brightness at zenith. The thick curves include the night sky contribution, while the thin lines indicate only the scattered component. The vertical dashed-dotted line marks the Sun zenith distance when the lower boundary layer height is 120 km. The upper scale indicates the lower Earth's boundary layer height in km.
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In the text

	Figure 2: Distribution of twilight observations in Alt-Az coordinates for TSF (filled symbols) and LSE (empty symbols). The astronomical azimuth has been replaced with the difference in azimuth between the telescope pointing and the Sun, .
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In the text

Figure 3: Zenith twilight sky surface brightness in the U passband from TSF (empty symbols) and LSE (filled symbols) with . The vertical dashed lines mark the end of nautical ( left) and astronomical ( right) twilight. The solid curve traces the simple model described in the text. The horizontal lines indicate the U dark-time night-sky brightness average value (dashed) for Paranal (Patat 2003a) and the 1 levels (dotted).

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In the text

	Figure 4: Same as Fig. 3 for the B passband.
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In the text

	Figure 5: Same as Fig. 4 for the V passband.
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In the text

	Figure 6: Same as Fig. 3 for the R pass band.
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In the text

	Figure 7: Same as Fig. 3 for the I passband.
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In the text

	Figure 8: Broad-band zenith twilight-sky colors. The curves have been computed using second oder polynomials fitted to the observed data. For comparison, the colors of the Sun are U-B=0.13, B-V=0.65, V-R=0.52 and V-I=0.81, while those of the night sky at Paranal are U-B=-0.36, B-V=1.03, V-R=0.74, and V-I=1.90(Patat 2003a).
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In the text

Figure 9: Twilight spectra obtained at Paranal at different Sun depression angles. For convenience, the spectra have been corrected to the corresponding V flux shown by the photometric data. Spectral resolution and sky patch differ from spectra to spectra (see Table 2). For presentation the spectra have been vertically shifted by the following amounts: +0.50 ( ), +0.35 ( ), -0.10 ( ), -1.0 ( ). The dotted line traces the solar spectrum, normalized to the continuum of the spectrum at 4500 Å.

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In the text

	Figure 10: Comparison between the zenith twilight-sky brightness measured at Paranal (small symbols) and the Crimean Astrophysical Observatory (large symbols) for the V passband.
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In the text

	Figure 11: Comparison between Paranal zenith sky brightness and the MCC++ model (Postylyakov 2004). The dashed curve traces the pure model solution without the contribution of night sky emission.
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In the text

Figure A.1: Geometry of the problem. For the sake of clarity, the observer's elevation has been exaggerated.


In the text

Figure A.2: Normalized density (solid line) and temperature (dashed line) profiles according to the MSIS-E-90 model (Hedin 1991). For comparison, the dots trace the values of the US Standard Atmosphere (McCartney 1976, Table 2.6).


In the text