in mag/arcsec$^2$ indicate darker skies (with less light pollution). Equation (\[eq:unihedron\]) gives the conversion from magarcsec$^{-2}$ to  and Table \[t:conversion\] lists selected pairs of corresponding values.

$$Luminance\ [cd/m^2] = 10.8\times10^4 \times 10^{(-0.4\ \times\ [mag arcsec^{-2}])}
   \label{eq:unihedron}$$

  mag/arcsec$^2$           comment
  ---------------- ------- ----------------------------------------
  14                   271 
  15                   108 
  16                  43.0 
  17                  17.1 
  18                  6.81 
  19                  2.71 
  20                  1.08 
  21                 0.430 
  21.75              0.215 value we assume for the [*overall*]{}
                           natural clear sky brightness
  22.0               0.172 value we assume for the [*zenithal*]{}
                           clear sky brightness

  : Conversion between mag arcsec$^{-2}$ and .

\[t:conversion\]

For the purpose of monitoring the night sky brightness over a long period of time, we use Unihedron’s Sky Quality Meter with an integrated lensing system. The model is called “SQM-LE”, but hereafter we will refer to it as “SQM”. All our devices are connected via ethernet. According to the manufacturer’s manual, the integrated lens narrows down the field of view to only $\approx$20$^{\circ}$ full width at half maximum (FWHM), thus at an off-axis distance of $\approx$10$^{\circ}$, the sensitivity declines by a factor of 2. At higher angles the response decreases rapidly, so that a point source located $\approx$19$^{\circ}$ off-axis contributes a factor of 10 less to the measured brightness than on-axis. 