Drogenberg is participating in an international project to measure light pollution. The TESS project originates from Madrid University and uses a photometer network.
Basically this photometer is installed on a rooftop and continuously measures the light. It sends it’s data towards a server in Madrid. It has been installed high, on a dark rooftop to avoid direct light from public lighting. Click on the pictures for a larger format.
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Although the sensor employed is the same that uses SQM, it uses a dichroic filter instead of the color filter. The overall response is thus different and fully includes the lines of the High Pressure Sodium (HPS) lamps since the spectral response is extended to the red (see TESS website). My expectations is that the values measured by the TESS photometer are a bit lower then the classic SQM Reader. I used a handheld SQM to measure the difference on a few nights. Within the range SQM 20-20,30 the TESS meter is about 0,15SQM lower (lower number, lighter sky). However by values better then 20,30 the difference seems larger (SQM measures better values).