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why is it OIII and SII anyway ?


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One for the particle physicists - why is it that the most common narrowband filters are Ha, OIII and SII ?

Ha totally understandable, Hydrogen being the most common element in the universe and Ha the first excitement state, but why oxygen and sulphur ?

As I understand it, it takes a catastrophic collapse of a pretty large star to produce elements beyond about oxygen (atomic number 8), let alone sulphur at atomic no 16.

Why don't we see filters for Helium, Nitrogen and Neon, all of which have visible light spectra, and the first two lighter than oxygen ? 

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One for the particle physicists - why is it that the most common narrowband filters are Ha, OIII and SII ?

Ha totally understandable, Hydrogen being the most common element in the universe and Ha the first excitement state, but why oxygen and sulphur ?

As I understand it, it takes a catastrophic collapse of a pretty large star to produce elements beyond about oxygen (atomic number 8), let alone sulphur at atomic no 16.

Why don't we see filters for Helium, Nitrogen and Neon, all of which have visible light spectra, and the first two lighter than oxygen ?

Ooooo! Interesting question, I've never actually thought about it and now I have I'm as puzzled as you.

I hope someone has the answer.

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i am no physicist but i recon the reason for a lack of nitrogen filter is to do with the amount of Nitrogen in our own atmosphere and it would probably cause interference,as for the other elements i could'nt even guess. just my thoughts  

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i am no physicist but i recon the reason for a lack of nitrogen filter is to do with the amount of Nitrogen in our own atmosphere and it would probably cause interference,as for the other elements i could'nt even guess. just my thoughts

From my limited understanding to emit light the nitrogen needs to be ionised. You can easily see the effect of photons released from ionised oxygen and nitrogen in our atmosphere if you go to see the Aurora Borealis.

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Done a bit of wikipedia-ing, quite interesting.

Toxic, I think you might have something on the Nitrogen - "Nitrogen also makes a contribution to visible air-glow from the Earth's upper atmosphere, through electron impact excitation followed by emission. This visible blue air glow (seen in the polar aurora and in the re-entry glow of returning spacecraft) typically results not from molecular nitrogen but rather from free nitrogen atoms combining with oxygen to form nitric oxide (NO)."  so I guess some of what we see on earth will get lost in sky glow.  I wonder if Hubble can see it.  It also emits in ultraviolet.

Helium emits a bright yellow and was first discovered as a spectrum line in sunlight during an eclipse.  Makes up 24% of all normal matter, so you'd think we'd see more yellow ??  Fun fact, Helium is one of the few elements with escape velocity built in, so if you pop a helium balloon, that gas will escape into space :shocked:

As for neon - "During cosmic nucleogenesis of the elements, large amounts of neon are built up from the alpha-capture fusion process in stars. Although neon is a very common element in the universe and solar system (it is fifth in cosmic abundance after hydrogen, helium, oxygen and carbon), it is very rare on Earth. It composes about 18.2 ppm of air by volume (this is about the same as the molecular or mole fraction), and a smaller fraction in Earth's crust. The reason for neon's relative scarcity on Earth and the inner (terrestrial) planets, is that neon forms no compounds to fix it to solids, and is highly volatile, therefore escaping from the planetesimals under the warmth of the newly ignited Sun in the early Solar System. Even the atmosphere of Jupiter is somewhat depleted of neon, presumably for this reason.

Neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or in high-voltage discharge tubes or neon advertising signs.[8][9] The red emission line from neon is also responsible for the well known red light of helium–neon lasers."

So you think we'd see more bright red from neon too, but maybe it gets blown away early from new stars by the solar wind

Must stop geeking, and do some work...

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You can get [NII] filters, but they have to be very narrow band to split them from Ha, which also has to be very narrow. In fact I have a set on order from Ian King. 3 nm Astrodons, at £385 each for 1.25" :eek: . I'm not sure why they don't make commercial He filters, perhaps there's no market.

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Ha is not the first excitation state, if I recall it is the electron dropping from n=3 to n=2. So in a way not Alpha as I would expect alpha to be n=1 to n=0.

OIII is about as bad as the term comes from the doubly ionised Oxygen which is O2+.

The term OIII seems to come from Chemistry OI being normal Oxygen, OII being O+ and OIII being O2+.

In effet one less ionisation state then the suffix would imply.

Not sure why it is not O, OI and OII - as in no ionisation and no I appended.

So blamed the chemists for the strange naming, quite at liberty to blame them for anything as it happens. :rolleyes: :rolleyes:

SII looks like it is named the same way SII refers to singly ionised Sulfur S+

CAnnot locate the actual shell changes that electrons go through for O and S, they are not obviously available.

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Ha is actually Balmer alpha with the electron dropping from n=3 to n=2. The true resonance line is Lyman alpha ie from n=2 to n=1 and is around 121nm in the deep UV. The Lyman forest is the heap of lines of various red-shifts that astronomers find in the spectra of quasars.

The [square brackets] around [OIII] and [sII] denote that these are forbidden transitions, which is why they are so faint.

I actually suspect is may have been spectroscopists who originated the I, II, III etc nomenclature.

BTW I am (Was) a chemist.

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Any of the NB filters we choose if aiming to be used to enhance RGB images must lie within the visible spectrum so I guess that eliminates quite a few. Ha squeaks in by the skin of its teeth.

The fact that an O111 filter makes a good nubular contrast filter indiciates that there's a lot of doubly ionized oxygen out there. It's particularly effective in certain cases, notably in the nebulae created by Wolf Rayet stars. (Crescent, Thor's Helmet, etc.)

Olly

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I assume it is a combination of wavelength (where the emission line is) and intensity (how much useful light there is). Neon for example has a strong emission line pretty much exactly where the sodium line (actually two lines) is. This makes the light useless because that's also where the light pollution is. Have a look at the following web-site.

http://astro.u-strasbg.fr/~koppen/discharge/

Cheers

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Actually it's the Helium that has a strong emission in the yellow. With a narrow enough bandwidth you could still separate them.

I see myself corrected - of course it's helium not neon. However the wavelengths are 1.4nm apart with the sodium line being 160 times stronger - no chance to separate.

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I think the key is that the main [NII] line is so close to the Ha line. You need a 3 nm filter to come close, and even then there's still some overlap. And 3 nm filters are a bit pricey.

Have a look here

http://www.astrodon.com/Orphan/astrodonfaqnarrowband/#h15

And here

http://www.astrodon.com/products/filters/narrowband/

And here for UK prices

http://www.iankingimaging.com/show_product.php?id=1014

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This is fascinating, I wonder what an image comprised of four or five emission lines would look like (would be interesting to process!).

Interesting to see the different elements having strong emissions in different parts of the spectra. Nitrogen has a strong emission in teal (a bit close to OIII) and blue, though carbon only seems to be of any use in green. Actually, does ionised carbon (CII) give off enough light to be seen by amateur telescopes? It is out there, but I cant see there being much of it - apart from star forming regions like M42.

http://www.esa.int/spaceinimages/Images/2009/07/Ionised_carbon_carbon_monoxide_and_water_in_star-forming_region

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I think the key is that the main [NII] line is so close to the Ha line. You need a 3 nm filter to come close, and even then there's still some overlap. And 3 nm filters are a bit pricey.

Have a look here

http://www.astrodon.com/Orphan/astrodonfaqnarrowband/#h15

And here

http://www.astrodon.com/products/filters/narrowband/

And here for UK prices

http://www.iankingimaging.com/show_product.php?id=1014

ouch - more expensive than my scope and mount together then ! 

They should make a slightly wider filter to take in Ha and NII together.  Presumably cheaper to make too.

I'm actually playing around with some Hubble data at the moment, since I've run out of my own data to process and can't get to the scope for a while.  Has something like 10 different filter wavelengths for the Catseye Nebula.  Will be interesting trying to figure out what each one is, and choosing a pallette to present them in.

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ouch - more expensive than my scope and mount together then ! 

They should make a slightly wider filter to take in Ha and NII together.  Presumably cheaper to make too.

That would be the Baader 7nm filter that most people use ;)

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Baader also make a Hb filter too, though as it will coincide with the Ha it won't give you any more data.

I'm thinking that a Ha, [OIII]. Hb image of eg M42 might give you the sort of image that you would be able to see visually if your eyes were more sensitive to colour.

Although the He line is very close to the Na D lines, in the absence of Na LP it might be viable. Actually the one pair of lines that are absent in HP Na lamps is the Na lines due to self-absorption by the Na.

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Granted, which is why I said it wouldn't give you more information. However if our eyes *were* sensitive enough to see true colour in M42 then a Ha, [OIII]. Hb might come close. Of course LRGB would be closer, but we're talking NB ATM.

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