I have continuous one minute


I have continuous one minute interval readings from my mast mounted networked SQL for six years, also all sky camera images at the same rate on the clearest of nights. Clear dark sky brightness continues to darken through midnight and beyond, by up to 0.3 magnitudes per square arc second. My sky brightness on good clear nights is 21.1. Gradual sky darkening during the night is always the case and is so across Europe. In general I do not do deep sky imaging until after midnight, for this reason. A combination of temperature drop towards dew point during the night with increasing mist on the horizon, blocking out distant city lighting and blocking horizon cloud reflection of city lighting beyond the horizon which causes most rural light pollution; also gradual turning off of lights after midnight, even if none local.