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Go Back   IceInSpace > General Astronomy > Radio Astronomy and Spectroscopy

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Old 18-05-2018, 09:10 AM
julianh72 (Julian)
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Question about atmospheric absorption (oxygen, nitrogen, etc)

This is probably a stupid question - but I guess I am doomed to remain stupid if I don't ask, but there is a chance for me to become a little bit less stupid if somebody can provide an answer ...

Given that our atmosphere is composed of a lot of nitrogen (78%), oxygen (21%), and a smattering of other "stuff", how is it that we are able to do ANY spectroscopic or narrow-band observation in the wavelengths corresponding to the major constituents of our atmosphere?

"Telluric contamination" is an issue at some wavelengths, but why isn't the absorption from the major atmospheric components so overwhelming as to render observation of OIII emission nebulae impractical, for example?
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Old 18-05-2018, 11:05 AM
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Well..
Maybe this link will shed some light on the issue...

BTW, interstellar OIII (triple-ionized) has very different absorption spectrum from molecular oxigen (O2) in the Earth athmosphere (which is also under much, much higher pressure here).

Some more on the subject can be found here as well.

Last edited by bojan; 18-05-2018 at 11:25 AM.
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Old 20-05-2018, 07:53 AM
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Thanks, Bojan - that helps a lot. (I guess life on Earth would have evolved quite differently if our atmosphere was comprised of 20% triple-ionised oxygen at very low pressure!)
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Old 21-05-2018, 01:03 PM
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Interesting Bojan but I wont pretend to understand of it at this point. Am still slack on learning/practicing spectroscopy, I keep saying I'll do more but you know....

Still I grab stuff like this you smart guys post so I can add it to my Evernote collection of material for when I get off my butt and dive deeper into spectra. So the contributions are not going to waste.
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Old 21-05-2018, 01:28 PM
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Hi sil,
It is simple:
At increased densities (and pressures) the spectral lines of gasses shift and widen... and that is why we have this "window" between UV and IR (BTW, our eyes evolved to be sensitive to light of wavelengths that propagate in this window. Please note, water is especially transparent to green light.. and the human eye is most sensitive exactly for that wavelength).
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Old 31-05-2018, 01:51 AM
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Atmospheric absorption in the visible range is significant and is a combination of specific absorption bands (mainly from O2 and H2O at the red end) and a general increasing absorption towards the blue end, from ozone , Rayleigh scattering (which is why the sky is blue) and scattering from aerosols. None of these are sufficient to completely absorb the light however but the effects do have to be corrected for. (Spectra should be corrected as though the observation was made at the top of the atmosphere) You can see a typical atmospheric absorption curve here
http://www.astrosurf.com/buil/atmosp...nsmission2.png
as measured by Christian Buil using his method described here
http://www.astrosurf.com/buil/atmosp...ansmission.htm

Cheers
Robin
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Old 31-05-2018, 02:54 AM
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Hi Bojan

Quote:
Originally Posted by bojan View Post
Hi sil,
It is simple:
At increased densities (and pressures) the spectral lines of gasses shift and widen... and that is why we have this "window" between UV and IR.
The reason for the window is because there happen to be no strong molecular transitions eg vibration or rotation) matching the energy range of visible photons. (The temperature of the atmosphere is too low for atomic transitions)

It is mainly the molecular absorption bands in the spectrum of Ozone molecules in the UV and of water molecules in the IR that define the limits of the window. I expect if the window had been at some other part of the spectrum, life would have evolved to exploit this region instead.

Cheers
Robin
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Old 31-05-2018, 09:44 AM
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I've always liked this broadband (across the solar spectrum) view of it showing what solar radiation arrives at the top of the atmosphere (inYELLOW and what gets through the atmosphere, to the earth, at sea level (in RED).

Best
JA
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Old 31-05-2018, 03:37 PM
julianh72 (Julian)
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Thanks JA and Robin - I'm feeling a little less stupid every day!

JA's image in particular helps - the absorption is indeed apparent, but there's enough getting through to the Earth's surface across the visible wavelengths to make vision (and back-yard spectroscopy) possible.
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