Calibration activity of a sky quality meter for autonomous light pollution measurement

The MINLU (Misuratore di INquinamento LUminoso) autonomous sensor suite has been realised and tested at University of Padova for monitoring light pollution from drones or sounding balloons. In particular, to acquire data on skyglow, two commercials Sky Quality Meters (SQM-LE) have been integrated in the measurement chain along with the downward pointing imaging subsystem. During the ground tests campaign before airborne measurement, both SQM showed an angular response deviating consistently from the expected performance and in particular the sensors’ field of view was larger than the expected one reported in the datasheet. This would have affected direct skyglow measurements during flight, as light sources close to the boundaries of the field of view would not be attenuated down but instead detected by the sensors. As a direct consequence the measurement of lowintensity skyglows could be affected by high-intensity punctual sources acting as lateral disturbances. So a dedicated test setup was conceived and realised to investigate SQM unit response to light sources in the field of view and identify the true angular response curve; the set up consists in a controlled rotatory stage moving the unit in front of a fixed diffusive light source. The work presents the calculated angular response curve underlying the presence of a larger than expected field of view and an unexpected double peak in the angular response, which is likely related to a non-perfect alignment of SQMs collimation optics.