Quote:
Originally Posted by gregbradley
Then when I started doing nightscapes I started seeing greenish tinges or even bright green sky's in some images. It's called air glow. It's either green (most common) or red depending on which chemical is reacting. Red is sodium.
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Hi Greg,
The fact that many people could see green tints in M42 when looking
through a telescope was a puzzle up until around 1927.
In fact it could also be detected through spectrographs as bands
around 500nm wavelength - in the part of the visible light spectrum we
perceive as green.
This was such a big mystery that it was speculated that there must be
some unknown new element which was coined "Nebulium".
Then with the coming of quantum mechanics the penny dropped.
It was not a new element at all. It was being created by electrons
in doubly-ionized oxygen (OIII) making transitions.
In order to reproduce it you need a particularly rarified atmosphere
such as in deep space. Or in the high-atmosphere of Earth where
the phenomena will cause the greenish tinge of airglow.
Airglow is the reason why even at the best dark sky locations the sky
is never completely black. When someone says, "I went to such
and such a place and outdoors at night it was so dark that you
could not see your hand in front of your face", I know it must have
been pretty cloudy
Who have become good at modelling airglow is Hollywood. More
specifically the computer animation profession such as the likes of
Pixar. In order to create realistic depictions of the night sky in the
movies, the software includes airglow modelling.
How we as humans perceive and process color is completely different
to how CCD's and digital photography processes color information.
For example, "white balance" in cameras is an engineering kludge
to compensate for the fact the set of transformations the camera performs
on the data is not like how our eye and brain does it but to try and
produce an outcome that looks similar.