Sky Quality Meter monitoring of the night sky at the Zselic Landscape Protection Area

Zoltán Kolláth, Konkoly Observatory, Budapest, Hungary

There are plenty of methods to estimate the quality of the night sky. Most of them are visual estimates of the limiting magnitude (the faintest star which can be visible by naked eyes) or the number of stars observable in a fixed region of the sky (e.g. a triangle or a square determined by bright stars). These methods work well to describe the expected view of the night sky; however, they depend on subjective factors, like the eyesight of the observer. To map and monitor the light pollution of the park and its neighbourhood it was necessary to select a portable photometer with high enough sensitivity to measure the subtle light of the rural night sky. A simple device, the Sky Quality Meter (SQM) by Unihedron was designed for this purpose, and it is used widely for light pollution monitoring.

SQM is a handheld device; it collects the light from a relatively large solid angle (1.5 steradian, approximately a cone with a 42 degrees half width). The device displays the average luminance of this solid angle in astronomical unit: magnitude per square arcsecond (mag/sqas). SQM is temperature calibrated and gives the luminance with the precision of 0.1 mag/sqas, which is equivalent to 10 percent in linear (cd/m2) units.

We have been monitoring the sky quality by SQM since January 27, 2007. All of our measurements were taken pointing the device to the zenith. The necessary conditions to perform a field survey were the following:

  • Moonless night.
  • No clouds or fog.
  • The Sun is at least 18 degrees below the horizon (astronomical twilight).
  • No direct light from artificial sources reaches the detector of the device.

The geographic co-ordinates of the measurements were registered with a precision of at least 100 meters. At normal conditions there are now detectable changes of zenith luminance on this distance scale. We used GPS device or a detailed topographic map of the region to get the geographic co-ordinates. The locations were recorded in Universal Transverse Mercator (UTM) co-ordinates. The UTM co-ordinates can be practical when one uses topographic map with this scale, and it also provides the distances between measurement locations easily. When needed, the UTM data are converted to standard longitude – latitude data.


Sky quality measurements in the Zselic Landscape Protection Area. The circles represent the luminance of the sky in a colour scale; the values are given in mag/sqas. The large continuous green area is the park; the brownish patches are the settlements in the area, the largest one is Kaposvár

The main conclusion of our measurements is that the local settlements have only a minimal effect on the quality of the sky. The luminance is only slightly increased in the vicinity of the small villages. The lights from the city of Kaposvár, north from the area mostly affect the sky background. Inside the Landscape Protection Area the quality of the night sky is better than 21.0 mag/sqas, typically around 21.3 mag/sqas. There is a clear increase in night sky luminance north from the park area, but acceptable conditions can be found for stargazing even at that location.

To display the quantitative results, we plotted the luminance of the night sky, as the function of the distance from the city centre of. For this plot we converted the SQM measurements to the standard luminance scale (cd/m2). It provides a more natural way to subtract the background sky brightness (L0), which represents the natural sky luminance and the overall sky brightness from distant artificial sources. With L0=300 μcd/m2 the distribution of measurements can be well represented with a straight line. This value agrees with our darkest measurements (21.5-21.6 mag/sqas) in the region. For this plot we selected observations close to the highway 67 south from Kaposvár. The observations from different nights are marked with different colours and symbols: red dots: 03/15/2007; green rectangles: 11/22/2008; blue crosses: 11/23/2008.


There is a clear overall trend in the sky luminance; the deviations from this trend are due to measurements close to local polluting sources, and in a less extent to changes in weather conditions (dust and humidity in the atmosphere). The artificial component of the measurements clearly scatters around a line with a slope of -1.3 on the log-log plot (dashed line), i.e. doubling the distance from the city centre results in ~2.5 times less artificial sky luminance

The horizontal lines represent the approximate borders for silver and bronze sky quality tiers recommended by the for dark sky parks. The artificial light contribution at zenith (as a fraction of natural background ~250μcd/m2 ) is 9.0 for the top and 3.0 for the lower lines. The regions between the two lines and below the bottom line represent the bronze and silver tier respectively.

Conclusion

We have performed sky quality (night sky luminance) measurements in the Zselic Landscape Protection Area and its vicinity. It is demonstrated that the Sky Quality Meter is capable of quick luminance measurements in remote locations. The pocket size, hand held device is especially useful for mapping the light pollution at multiple locations during a one night long monitoring session. The main light pollution source in the park is the city of Kaposvár. The local settlements have only a very limited effect on night sky quality. Inside the landscape protection area the quality of the sky everywhere reaches the silver tier recommended by the International Dark Sky Association.