After a few weeks of persistent weather with cirrus clouds, past weekend, 17apr2015, brought an extensive dry polar air mass over my home (“Windhoek”, because there is no spot the wind does not touch, and a quip after the famous night-sky wonderland of Namibia). The kind that often makes springtime such an oppertune time for stargazing.
The clear skies gave an excellent oppertunity to gauge the quality of the sky over our new house, which is located in a rural area some 10km away from two small cities (100k people). The immediate vicinity (~2km) is virtually clear from light sources, as the neighbors (almost all of whom are farmers) do not unnecessarily light their yards.
To this end, I determined the size and amount of city lights on the horizon, star counts, SQM readings, and photograhic examination.
Light pollution domes
S-SW: By far the largest dome is Deventer (10km, 100k pop) , and spans a 70-degree azimuthal swath reaching up to 40 degrees elevation. The eastern edge of the dome starts right past the meredian. The dome is yellowish / orange, with a clear bright spot exactly where the city center is; with massive sky-pointed lights that shines on a medieval church complex.
N: A smaller dome is caused by Raalte (8km, 20k pop)in the North. It spans about 20 degrees in azimuth, but also reaches up to 30~40 degrees. Contrary to Deventer, the appearance is bright white, almost like zodiacal light.
There are some trees in the distance along the eastern hemisphere. In the west, there is clear view up to the horizon. I could easily see the moon, mars and mercury at 3 degrees altitude with binoculars.
Apart from the North and South, the rest of the horizon is pretty decent with many small domes that pose no threat, all of them being below 10 degrees. Especially the E-SE is very nice. The line of sight does not pass any populated area in that direction for 30 to 40km.
Finally, the zenith is best. The zenithal dome of about 75 degrees (extending to the South-east) is very dark, with no apparant light pollution. Hundreds of tiny stars, just beyond direct vision, are simmering there. It kind of resembles a dark area in the French countryside.
The milky way, rising before dawn up to 40 degrees, showed some detail. It could be seen to stretch from Cassiopeia in the north down to the Scutum cloud in the south-east. The dark rift was pronounced, and the North America Nebula standing out clearly. The cygnus/scutum split of the milky way was also noticable, and some hints of structure in the southern milky way was visible.
M13 could be seen unaided (barely) at 45 degrees elevation in the east, but becomes very easy to see once overhead just before dawn.
On two nights, I measured the V-band flux in a 20-degree cone using a calibrated Unihedron SQM sensor (the SQM-LE version, link). I measured the readings manually, one time in the evening when the sun went below -18 degrees (2100UT) and one time before dawn (0100UT). Depending on wether Vega or the milky way crossed the view, the readings varied between 20.84 and 20.91
Once the weather station is set-up, this site will collect permanent and continuous readings.
Using a star chart, you can count the stars you can barely see in a predefined area (link), and this is a measure of the NELM. For this method, I exclusively looked at the very zenith. To prepare, I dark-adapted for at least 1 hour, and sat in a dark corner with dark sheds around so that I only saw the zenith, without being influenced by the bright domes in the North and South. On two nights, I counted 3 different areas. Again around 2200UT, and 0100UT. Every area I counted at least 2 times, at different times. I counted areas Q, I and F (see pdf in the link), with the results an indicated NELM of 6.4, 6.5, 6.5 giving an average of 6.5 zenithal NELM.
Usually subtle light pollution shows up in a picture, even when you don’t see it visually. So I took a picture of Deneb region, which was rising in the east at an altitude of about 40 degrees. Imaged with 24-105mm F4 at 6400ISO, raws, unmodified Canon 6D at 105mm/F5.0 stack of 39 images of 60s exposure. Levels adjusted and 50% crop. The top looks slightly more yellowish. But this is more or less the natural milky way color, so I do not think any light polution is obvious. Besides, the top is higher in the sky, with less influence by the pollution.. I’m very happy with this finding!