|
Thanks Aiden - I have read the excellent Wikepedia article on Rayleigh scattering and although I got lost in the maths part I think that the third para in "why the sky is blue" gives an important clue and I think this goes a long way to answering my question. It points to the fact how sunlight reddens as it sets because it passes through denser atmosphere and the scattering removes most of the blue light directly reaching the earth bound observer.
I suppose with very humid NE winds that the presence of a high level of water molecules has a similar effect to the above and reduces the blue
light reaching the observer.
As I have explained, I observe in a high light pollution area and if I try to "stretch" dim nebulosity (even red) it comes to a point where I get a whitening effect where I think that the light pollution level starts to exceed the level of the dim nebulosity.
Also, I have had a critical look the way I am imaging. I notice that I have the gain setting on my QHY5 set far too high to capture a full range of nebulosity.
So in future I will set my QHY8 gain to a lower level and try to avoid
imaging blue nebulas on high dew nights.
Cheers Peter
|