In the previous post you may check what LowSpec spectroscope is, and how does it look. Since the conditions to real first light on stars are still not satisfied, I decided to capture spectrum of the night sky background. The idea behind was to record and analyse the composition of light pollution. I recorded two spectra over two different nights. First one was made during 100% cloudy night and with Moon present. Second one during relatively clear night – I estimated NELM to 4.5mag. Some haze and high clouds were present.
I did not attach any optics to the spectroscope. I just pointed the device input to the sky. Together with 2″ nosepiece the apparent field of view was maybe about 30 degrees, so the measurements were averaged. I live in suburban area, so there are many street lights in the area, but also pretty lot of household lights. I recorded 120 minutes of data in 3 minutes subframes. Raw frames were calibrated and stacked in Maxim. I performed spectrum processing in BASSProject software – calibration was done with Relco SC480 starter.
Two most obvious features are wide absorption line at 5890A and emission line at 5688A. Both lines comes from sodium (Na) and its source are high pressure sodium lamps widely used as street lights in my location. There are also few mercury (Hg) lines that come from another type of street lights. Natural airglow at 5577A (oxygen based) is barely visible at the spectrum made during cloudy night. But it is obvious under clear sky. The small signal at the blue part of the spectrum may come from white LED lights ( https://www.nature.com/articles/lsa2015105/figures/5 )
There are two absorption lines visible in the spectrum made under cloudy sky – these are hydrogen alpha and beta lines. These features do not come from light pollution sources. They come from the Moon light dispersed in the cloud. Moon reflects sun light, and these absorption lines are actually sun spectrum features 🙂
Looking at that spectrum one may try to figure out the way to fight light pollution using filters. But this is not an easy task, when light pollution contains wide lines, like that from high pressure sodium lamps, or (even worse) from white LED lights. These cannot be filtered out without sacrificing large amount of valuable signal. The actual sources of light pollution may be different at different locations. It may be useful to be aware of them, but no filtering will compensate for the dark skies 🙁
Cover photo by Sandrachile on Unsplash