of measurements the differences of both device data readings lie within an interval of only $\approx$0.1 . But it was also recognized that under certain circumstances scattered moonlight caused differences of up to $\approx$0.9. A discussion on that topic can be found in the Appendix.

Neglecting scattered moonlight values, the results show that the initial calibration of our SQMs is offset by only $\approx$0.046 . Taking that offset into account the relative accuracy between the two devices is given by the standard deviation of all data points. Fig. \[fig:qualityhistogram\] shows the distribution of the calculated differences ($\Delta$) binned to 0.04 . It can be seen that after rejecting data with scattered moonlight for 87.5 % of all data points the difference in the response of our devices is equal to or lower than 0.08  leading to a standard deviation of $\approx$0.04  which corresponds to a relative error of only 4 %. Under the assumption of a 3 % growth of the NSB for Vienna, we will be able to detect a significant ($2\sigma$) change of the NSB level after 3 years (by the end of 2015).

So far the SQM appears to be a handy and robust instrument for long 