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

In the last decade, numerous sky quality meters (SQMs) have been installed around the world, aiming to assess the temporal change of the night sky brightness, and thus the change in light pollution. However, it has become clear that SQM readings may be affected by ageing effects such as degradation of the sensor sensitivity and/or loss of transmissivity of optical components (filter, housing window). To date, the magnitude of the darkening has not been assessed in a systematic way. We report for the first time on the quantification of the SQM ageing effect and describe the applied method. We combine long-term SQM measurements obtained between 2011 and 2019 in Potsdam-Babelsberg (23 km to the south-west of the centre of Berlin), Vienna and Stockholm with a readily available empirical twilight model, which serves as a calibrator. Twilight SQM observations, calibrated for changing Sun altitudes, reveal a linear degradation of the measurement systems (SQM + housing window) with the following slopes: 34 ± 4, 46 ± 2 and 53 ± 2 mmag_SQM arcsec^-2 yr^-1 for Stockholm, Potsdam-Babelsberg and Vienna, respectively. With the highest slope found in Vienna (latitude ∼48°) and the lowest one found in Stockholm (latitude ∼59°), we find an indication for the dependence of the trend on solar irradiance (which is a function of geographic latitude).