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SII: is it worth it?


Epicycle
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Hi,

I guess this is a question that must've come up in the past and I'm hoping those of you more experienced should have a view.

Is it worth getting a SII filter for UK skies? Part of the argument against it is that there are ways of creating a synthetic SII

from Ha and OIII data, and one could use the time for grabbing more subs in these two more prominent lines emissions.

Ideally one would like to see a good amateur image where genuine SII made a real difference. It's difficult to judge this just from looking at the image in the sense that something quite similar could've been achieved from suitable processing

of just the ha and oiii data...

Anyway, thoughts on this welcome as these filters are not cheap!

cheers

Epicycle

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If there is a way of creating S11 from Ha and O111 I don't know it. ( There are quite a few things I don't know :) ) Are you referring to Hb ? It does a similar but not exact job.

S11 is good for S11 targets under any skies I think.

As a quick example of Ha and S11 here is an Ha / S11 / S11 image. Short on S11 I'll grant you but all the blue bits are S11.

Dave.

post-493-0-72496500-1372601332_thumb.jpg

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Dear Ant and Dave,

those are awesome pictures! Thanks for the answers. Perhaps I should quickly clarify myself: one can't *really* replace SII, but just create a fake

one with reasonable results like doing in PI sqrt(Ha*Oiii) and use that for one of the color channels.

Perhaps the question should then be: if you were to begin doing SII-imaging, which targets are worth it?

cheers

E.

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Are you referring to Hb ? It does a similar but not exact job.

H-beta emission traces the same material as H-alpha but with about a third of the emissivity as the Balmer beta 4-2 transition is weaker than the alpha 3-2 transition for almost all conditions you'll encounter. One advantage is that the H-beta line is in the blue/green, so you can capture signal with sensors that have very poor red sensitivity (which would also be no good for [s II], which is slightly further red than H-alpha). But if red sensitivity isn't an issue there's no sense in using H-alpha and H-beta.

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Dear Ant and Dave,

those are awesome pictures! Thanks for the answers. Perhaps I should quickly clarify myself: one can't *really* replace SII, but just create a fake

one with reasonable results like doing in PI sqrt(Ha*Oiii) and use that for one of the color channels.

Perhaps the question should then be: if you were to begin doing SII-imaging, which targets are worth it?

cheers

E.

I told you there were quite a few things I didn't know.

Dave.

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H-beta emission traces the same material as H-alpha but with about a third of the emissivity as the Balmer beta 4-2 transition is weaker than the alpha 3-2 transition for almost all conditions you'll encounter. One advantage is that the H-beta line is in the blue/green, so you can capture signal with sensors that have very poor red sensitivity (which would also be no good for [s II], which is slightly further red than H-alpha). But if red sensitivity isn't an issue there's no sense in using H-alpha and H-beta.

I read a discussion that became heated some time ago re Ha and Hb. The resultant colour wasn't red !

I was referring to creating a false Hb from Ha and O111 rather than S11 from Ha and O111.

Dave

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You can't simulate SII . You can blend your Ha and OIII to create another colour channel which you can then use in order to create a false Hubble pallette effect, but SII emission isn't in exactly the same physical region as either of the other 2 filters, in the way that H-beta occupies the same region as the Ha, so simulating the colour pallette isn't the same thing.

In answer to the question of 'is it worth it?', this is up to you to decide. There aren't a huge amout of targets with good SII emission, but the ones there are definitely benefit from it......the Crab and Dumbell nebulae being two that come to mind straight away.

I hardly ever use my SII filter, but wouldn't get rid of it.

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The resultant colour wasn't red !

Not sure what you mean? H-alpha is in the red, H-beta is blue/green, but you usually map narrowband to RGB channels which don't necessarily match their natural wavelengths (e.g. in the Hubble palette the red H-alpha line gets mapped to the green channel).

I was referring to creating a false Hb from Ha and O111 rather than S11 from Ha and O111.

I think you mean creating a false third channel? [O III] (not O111, by the way) doesn't really have much to do with H-beta emissivity, so H-alpha + [O III] doesn't directly correspond to anything as such (although arguably H-alpha coupled to gaussian noise would correspond to H-beta). The H-beta line is effectively H-alpha with a third of the signal due to the reduced emissivity (based on the tables in Astrophysics of Gaseous Nebulae and Active Galactic Nuclei). Doing H-alpha/H-beta/[O-III] doesn't make much sense as two Balmer channels trace the same material but at different signal-to-noise.

Edited by Ben Ritchie
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You can't simulate SII . You can blend your Ha and OIII to create another colour channel which you can then use in order to create a false Hubble pallette effect, but SII emission isn't in exactly the same physical region as either of the other 2 filters, in the way that H-beta occupies the same region as the Ha, so simulating the colour pallette isn't the same thing.

In answer to the question of 'is it worth it?', this is up to you to decide. There aren't a huge amout of targets with good SII emission, but the ones there are definitely benefit from it......the Crab and Dumbell nebulae being two that come to mind straight away.

I hardly ever use my SII filter, but wouldn't get rid of it.

Fair enough Rob. Any noteworthy widefield object for which SII is strong?

cheers

E.

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Not sure what you mean? H-alpha is in the red, H-beta is blue/green, but you usually map narrowband to RGB channels which don't necessarily match their natural wavelengths (e.g. in the Hubble palette the red H-alpha line gets mapped to the green channel).

I think you mean creating a false third channel? [O III] (not O111, by the way) doesn't really have much to do with H-beta emissivity, so H-alpha + [O III] doesn't directly correspond to anything as such (although arguably H-alpha coupled to gaussian noise would correspond to H-beta). The H-beta line is effectively H-alpha with a third of the signal due to the reduced emissivity (based on the tables in Astrophysics of Gaseous Nebulae and Active Galactic Nuclei). Doing H-alpha/H-beta/[O-III] doesn't make much sense as two Balmer channels trace the same material but at different signal-to-noise.

There must be a degree of confusion here. I was answering the OPs original question with another question. IE Did he mean make a synthetic Hb as has been done by many imagers far superior than myself. I am not aware of being able to make a synthetic S11 as the S11 is more often than not in a different place to the Ha and O111.

As for the discussion I read regarding the colour of Ha and Hb combined, I took no part in it, was not that interested in it and one of the participants appears to have withdrawn from posting ever since.

I use 1 instead of I out of a bad habit. I'm not likely to change :)

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I took some time before I bought the SII filter but I have found some nebulae with useful amounts of SII emission. Been trying to think which... The Eagle contains SII which is different from Ha or OIII. Both OIII and SII are very much fainter than the Ha though. I think the Heart Nebula was also one with significant SII.

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