Night sky spectrum

[drjolo]

I was not yet able to point my LowSpec spectroscope to stars, but Saturday evening I pointed it to the cloudy night sky to measure and identify main light pollution sources. LowSpec has 600 l/mm grating, and camera used was QHY163M. 

Main source of LP at my location are high pressure sodium lamps, that are responsible for emission Na 5688A and  wide bell shaped 5890A line with absorption peak. There are also few weaker lines that comes from mercury Hg - these are emitted by different type of street or garden lamps. Natural airglow that comes from oxygen occurs at 5577A and 6300A. First of these lines can be identified, however I was not able to detect anything meaningful at 6300A.

There are also two obvious absorption lines at 4861A and 6563A - I quickly identified them as hydrogen beta and alpha. But I had not idea what could be the source of such lines in the spectrum of night sky background. After some time I remembered that night before I admired almost full Moon in the sky. And Moon reflects Sun light, so these two lines comes probably from Sun light reflected from Moon and then dispersed in the cloudy night sky.  Barely visible magnesium triplet confirms this. However I plan to take night sky spectrum two more times when Moon will not be visible - with and without clouds. 

Edited January 13, 2020 by drjolo

[drjolo]

Two days ago I collected night sky spectrum as well, but this time without clouds and without Moon. I estimated NELM to 4.5mag - some haze and high clouds were still present.

Hydrogen absorption lines that came from sun light disappeared due to missing Moon. Oxygen glow at 5577A is much more visible. Also I adjusted focus much better, so now Na doublet at 6154A is resolved, and also some peak appeared inside wide Na absorption line at 5890A that comes from high pressure sodium lamps. Small peak around 4500A probably comes from blue part of white LED light https://www.nature.com/articles/lsa2015105/figures/5 .