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Old 12-01-2016, 02:05 PM
pjphilli (Peter)
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Poor Blue Nebulosity

Hi

I have a Qhy8 camera which I use for deep sky imaging in a NW suburb (Thornleigh) of Sydney. As usual sky visibility is pretty poor even on the best of occasions and not helped by a nearby major road with sodium lamp lighting. I use a neodymium light pollution filter.

I notice that imaging of feint blue (reflection?)nebulosity is consistently worse than feint red (emission?) nebulosity. Last Sunday night I imaged
the Running Man nebula which has a large proportion of blue nebulosity.
I notice that I could only capture the more dense blue nebulosity.
Although the sky was clear of clouds there was a NE sea breeze blowing
and the dew was pretty fierce (not uncommon from this wind which
blows from the sea).

I would like some comments on this phenomenon. Maybe the very moist
atmosphere absorbs the shorter blue light more than the longer wavelenth
red light?

Cheers Peter
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Old 12-01-2016, 02:30 PM
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Somnium (Aidan)
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Rayleigh scattering ??? perhaps. https://en.wikipedia.org/wiki/Rayleigh_scattering
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Old 13-01-2016, 11:04 AM
pjphilli (Peter)
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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
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