NELM and SQM in late March in Sodankylä, Finland

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!


3 thoughts on “NELM and SQM in late March in Sodankylä, Finland

  1. Colin Hawke September 27, 2011 / 10:16

    I curious how you are estimating NELM as your numbers seems to be a bit higher than what the SQM numbers suggest. For example a NELM of 5.5 is around 20 on the SQM (mags per arcsec squared). Most people who estimate NELM visually (Ursa Minor W-Umi technique) usually underestimate rather than overestimate due to their eyes being not dark adapted enough.Clear Skies, Colin (another geoscientist – actually geophysicist).

  2. Juha Ojanperä October 18, 2011 / 19:11

    Hi, and thanks for your comment! In this project I estimated NELM just by taking a random area of the sky that is at least of altitude 50 and that is free of light pollution. Then I just sketched the stars that I could see, and then I checked afterwards from Sky Map Pro, which was the faintest star that I was able to see. Nowadays I'm still using the same technique but now I mostly use Ursa Minor as the constellation where I draw the stars I can see.

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