term studies of the development of the nightsky brightness. Under some circumstances scattered moonlight can influence the readings, therefore some sort of shielding should be used for the lensed SQM version.

![Histogram showing the distribution of the differences between the measurements of 2 SQMs operated in parallel mode. Red lines mark distribution before correction for scattered moonlight. The bin size is 0.04 .[]{data-label="fig:qualityhistogram"}](Figure2.eps){width="13cm"}

Spectroscopic measurements
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The spectroscopic observations, presented below, were done with a 80cm Cassegrain telescope in the North Dome of the Vienna University Observatory. Its focal length amounts to 6.7m. A spectrograph ’DSS-7’ produced by SBIG was attached to the telescope. DSS-7 is a grating spectrograph offering five slits with different widths, the narrowest of which provides for a dispersion of $\sim0.55$nm/pixel, corresponding to a spectral resolving power of $\lambda$/$\Delta$$\lambda$ $\approx$ 500. An ST-7 CCD camera, also made by SBIG, was connected to the spectrograph to acquire the spectra. With this camera, the wavelength coverage is approximately 450–850nm (with some small variations between the observing runs). The chip of the CCD camera was cooled down to -10$^{\circ}$C during the observations.

To record the spectra of the night sky, the telescope was pointed to the south or to 