I started making regular observations of NELM and SQM in late March last year when I bought a SQM-L device for myself. Then I started a project about collecting these measurements and comparing NELM vs. SQM. Here’s a preliminary graphical presentation of my results. In this graph, there is SQM plotted vs. NELM. This dataset is a compilation of 31 measurements. I also fitted a trend line in the data, and produced an equation for the line which is like this:
This work will be going on and I will be collecting more data to make more solid basis for making conclusions. I have been using Open office Calc to produce this graph and trend line.
During late March I made some observations of naked eye limiting magnitude and SQM -measurements. My observing place was Kaanaanmaa neighbourhood in Sodankylä town, in northern Finland. The observing place is semi-urban and there are street lights causing direct light pollution. The light pollution glow caused by Sodankylä town is evident in south. East, North and zenith are relatively dark (I got some SQM readings in NE sky around 20,00 mag/arc sec). When I made my measurements, Moon was not in the sky, and there were thick cover of snow on the ground. I made these measurements from zenith. I made also estimations of seeing, backround sky darkness and of transparency (scale from 1-5, 1 is best).
These measurements are just preliminary data, and it is not possible to make any reliable conclusions based on this. But anyway, I could make some notes from this data:
1. I made the first and last observations in a parking lot, which is relatively well protected from direct light pollution. Other measurements were made without any direct light pollution shelter. In the sheltered parking lot (there are trees around the parking lot that are acting as a direct light pollution protection) I got slightly better SQM values than in the other spot. So it is good idea to make your measurements in a spot that is protected from direct light pollution!
2. When NELM is around 5,4 (mean value of my measurements) SQM is around 18,68 (mean value of my measurements)
Here you can see my observations as a table:
And as a graphical presentation:
Now the sky has already got too light here up north, that it is not possible to make these mesurements before autumn. But more observations well be coming then!