Correcting sky-quality-meter measurements for ageing effects using twilight as calibrator

In the last decade numerous Sky Quality Meters (SQMs) were installed throughout globe, aiming to assess temporal change of night sky brightness (NSB), and thus in light pollution. However, it has become clear that SQM readings may be affected by aging effects such as degradation sensor sensitivity and/or loss transmissivity optical components (filter, housing window). To date, magnitude darkening not been assessed a systematic way. We report for first time on quantification effect describe applied method. combine long-term measurements obtained between 2011 2019 Potsdam-Babelsberg (23 km southwest center Berlin), Vienna Stockholm with readily available empirical twilight model, which serves calibrator. Twilight observations, calibrated changing sun altitudes, reveal linear measurement systems (SQM + window) following slopes: 34$\pm$4, 46$\pm$2 53$\pm$2 milli-mag$_{\rm SQM}$ arcsec$^{-2}$ yr$^{-1}$ Stockholm, Vienna. With highest slope found (latitude $\sim$48$^\circ$) lowest one $\sim$59$^\circ$), we find an indication dependence trend solar irradiance (which is function geographic latitude).