reported to be close to 18.9  as well. According to Fig. \[fig:histo\], almost 90% of all nights at IfA are characterized by mean zenithal NSBs larger than the “” threshold: during the vast majority of all nights, therefore, entirely rod-dominated vision as well as Milky Way visibility is not reached.

At our rural, 880m-above-sea-level observing station LFOA, in contrast, only 20% of all nights have mean zenithal NSB values brighter than 18.9 . The full range of mean NSBs at LFOA is 17   to 21 . Note that even the latter value is still about 1 magnitude (or a factor of 2.5) above the natural zenithal night sky brightness.

![Histogram showing the relative (top) and cumulative (bottom) distribution of the mean NSB measured at IfA in Vienna (black dashed bars) and LFOA (red solid bars). The vertical line indicates the limit between scotopic and mesopic vision at .[]{data-label="fig:histo"}](Figure8a.eps "fig:"){width="11.5cm"} ![Histogram showing the relative (top) and cumulative (bottom) distribution of the mean NSB measured at IfA in Vienna (black dashed bars) and LFOA (red solid bars). The vertical line indicates the limit between scotopic and mesopic vision at .[]{data-label="fig:histo"}](Figure8b.eps "fig:"){width="11.5cm"}

Spectral lines detected in the Night Sky
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Figure \[fig:skyspectrum\] shows 