From: Martínez-Ledesma et al., Spectral and Zonal Restriction Justification
The Dark Sky Mandø project is measuring and assessing the darkenss levels on Mandø primarily through direct measurements, but also indirectly through image analysis. The direct measurements are all made with variants of the Unihedrom SQM-L Sky Quality Meter. The Dark Sky Mandø project has 3 SQMs at our disposal, which each serves different purposes, which is described below.
The actual measuremnt sensor is the TAOS TSL237S sensor which translates the light levels to a frequency, that the SQM's microcontroler again converts to an output in units of mag/arcsec2 (magnitude per arc-second squared), which often is abreviated mpsas. Night time values are in the range of 16 to 22 mpsas, where 22 mpsas only is obtainable in the darkest areas on Earth and the higher a value, the darker. A comversion from the mpsas unit to the more human relatable Bortle scale can be found in this figure:
The L in the SQM-L devices indicate that they are equiped with a lens, which means the angular sensitivity is increased drastically, such that it has a FWHM sesitivity of 20° and at a width of 38° it is only measuring 10% as can be seen on this graph by Unihedron. All measurements are, unless stated otherwise, directed towards the zenith.
The handheld, mobile device is an Unihedron SQM-L which gives a direct readout on a display and is thus an ideal device for quick assessments of the darkness.
The bulk measurement device is an Unihedron SQM-LU which has no display, but instead can be read though a USB connection to a computer. Our device has been fittet with a mounting kit, which enables the mounting of the device on a car. This makes it possible to make bulk meassurements all over Mandø in a convinient way. The measurement computer is connected to a GPS device, which is mounted on the car's roof such that each SQM measurement is acompagnied by an acurate location.
In the town of Mandø we have permanently mounted an Unihedron SQM-LU in a weather proof housing. The device is connected to a Raspberry pi computer, which measures an uploads a measurement every minute. These measurements are presented online in real time, to give a live asessment of the Mandø darkness. The measurements are stored in a database for subsequent statistical analysis, which is presented on this website.
