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What is the contribution of SII filter?

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#1 Sreesha

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Posted 05 February 2025 - 12:12 AM

I am fairly new to mono imaging (I have ASI 2600mm and Optolong 2 inch, 3nm LRGBSHO filters), and need some clarification.

 

I understand Ha filter covers the region that is usually Crimson/Red. 

And Oiii covers the region that is usually Cyan/Bluishgreen.

 

What does Sulpher II filter do in the same parlance?  While I know it also filters certain frequency of Crimson/Deep Red - is that the fundamental reason to capture Sulpher II?  Or is it that it is mixed with Oiii channel in post processing to produce Yellowish hues?

 

While I know that coloring is a personal choice, and there are different pallettes, my clarification is specifically to understand the contribution of what we do with SII filter and which colour is produced?

 

 

Also, please see attached image.  How will know for a subject like this whether Ha and Oiii is good enough or Sii is also needed?  

Sometimes RGB brings the colours (eventhough it is predominantly looked for stars) of nebulae too, and so how do we decide between what RGB gives vs the SHO filters if RGB can also bring nebulae colours? 

 

Am really confused.

 

Thanks

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  • sample1.jpg

Edited by Sreesha, 05 February 2025 - 12:13 AM.


#2 acrh2

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Posted 05 February 2025 - 12:40 AM

Ha filter's purpose is to isolate hydrogen gas from the rest of the chemical elements to scientifically study chemical composition of DSOs. You use it to make pretty pictures, so you can do whatever your heart desires. 

 

People add Sii as red to their SHO images to see it as a separate color, to make more interesting images. But because Sii is usually the least abundant of the 3 elements, it's usually showing up as red/golden highlights on top of the Ha signal. You can see it in any number of SHO images on astrobin. 

 

Here's a good example of HSO vs. SHO (vs. HOO,) where all 3 signals are about the same strength, and some parts of the DSO have those 3 signals almost entirely separate from the other 2. You can clearly see how different colors of elements mix and create hues:

 

53990679466_be9990b67f_z.jpg53105977377_6bf8af1606_z.jpg

51826667669_84cf9184e9.jpg

 

 

 

 

 

 

 


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#3 Sreesha

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Posted 05 February 2025 - 12:47 AM

Got it, thanks.  

 

So one reason to capture SII is to demark Ha from it by coloring it yellow (RGB <=> SHO).

 

But by looking at an image how can we decide if Sii is needed?  just because there is yellow in an image - does it mean it would benefit from Sii (if we wanted the yellow)?

in other words, i may see just reddish hues in an image, and so i may think just Ha is enough.   but i wouldn't know if the region has a portion that can be made yellow by using Sii data.

other way of seeing it:  is there tool that tells me the ratio of Sii (vs Ha) in a nebula?  then that answers my question.  because based on sii concentration (as compared to Ha), i can decide if Sii is needed.

astrobin images are surely a reference, but given that coloring is a personal taste, it would be incorrect to judge looking at the filters used vs reality  of what the nebula contains.

all this confusion arising from one need:  i don't want to waste capturing Sii if it has no significance to image colour rendition (particularly yellow hues).

a region like IC 443 (jellyfish complex) is relatively easy to understand for Sii need because we have seen several samples, but that is just a obervation.  but there must be a methodical way to understand if the nebula / region demands sii data.

 

 

 

 

 

 

Ha filter's purpose is to isolate hydrogen gas from the rest of the chemical elements to scientifically study chemical composition of DSOs. You use it to make pretty pictures, so you can do whatever your heart desires. 

 

People add Sii as red to their SHO images to see it as a separate color, to make more interesting images. But because Sii is usually the least abundant of the 3 elements, it's usually showing up as red/golden highlights on top of the Ha signal. You can see it in any number of SHO images on astrobin. 

 

Here's a good example of HSO vs. SHO (vs. HOO,) where all 3 signals are about the same strength, and some parts of the DSO have those 3 signals almost entirely separate from the other 2. You can clearly see how different colors of elements mix and create hues:

 

53990679466_be9990b67f_z.jpg53105977377_6bf8af1606_z.jpg

51826667669_84cf9184e9.jpg



#4 PIEJr

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Posted 05 February 2025 - 12:53 AM

Shoot a series with the SII filter in it. Stack it, Process it, see if you like it.

 

Shoot the same series without the SII in it. Do the exact same thing. See which you like more.

 

I always just went with all the prescribed filters. LRGB the first night. Ha, SII, OIII the next.

 

I did that for 3 years, me and my mono, my 8 position ASI EFW, many, many nights.

 

I wound up with a great collection of images!

Albeit, all B&W images. But they sure looked very detailed.

 

Now that I have your leg and I'm pullin on it, Try this read.

I simply typed in, "What does an SII filter do in Astrophotography?"

 

Sometimes you have to be careful what you ask for. wink.gif  waytogo.gif


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#5 acrh2

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Posted 05 February 2025 - 01:00 AM

 

Also, please see attached image.  How will know for a subject like this whether Ha and Oiii is good enough or Sii is also needed?  

Sometimes RGB brings the colours (eventhough it is predominantly looked for stars) of nebulae too, and so how do we decide between what RGB gives vs the SHO filters if RGB can also bring nebulae colours? 

 

 

If there's Sii signal, then your image will benefit from having it because it will add extra details. The problem is that Sii is usually very weak, so it's a personal decision whether you want to include it or not, or how much time are you willing to spend on collecting the data.

 

These are the sources that I use to determine if there's enough Sii signal to make it worthwhile:

https://www.galactic...er-expectations

https://cosgrovescosmos.com/

They have mono images of SHO data normalized to show their relative strength for many different popular DSOs.

 

Here's a pretty good example (with strong-ish Sii) that demonstrates the effect of adding Sii to basically an RGB image:

51995229302_bff7481fd2_z.jpg 53321562137_1547f394a7.jpg

 

Ignore the blue colors in the second image - they should be in the first image as well, but this Oiii signal is very very weak, so it was not accessible to me at the time.

You see uniform red color of Ha in the first HOO image. But the second SHO image has a mix of the green of Ha and red of Sii, so it creates those red hues at the bottom on top of the green-yellow Ha signal. It's your preference which one you like better.


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#6 dswtan

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Posted 05 February 2025 - 01:17 AM

So one reason to capture SII is to demark Ha from it by coloring it yellow (RGB <=> SHO).

Not "one" reason. That IS the reason. But not to color it "yellow". You can color it whatever you like. In SHO, it is red. But with more sophisticated color mixing, you can do any mix you like. This is some of the joy and freedom of narrowband imaging.
 

But by looking at an image how can we decide if Sii is needed?

If you captured Sii, then you can try it in your image. There are no rules. You are painting with a palette of your choosing.

If you didn't capture it and you're looking at your processed image with just Ha and Oiii captured, then it's too late -- or you have to set up again and capture Sii later. A number of objects come out quite well with just Ha and Oiii. I find Astrobin helpful here.
 

here must be a methodical way to understand if the nebula / region demands sii data.

There is not. You can read up about the object and use previous images like Astrobin that you also mentioned. acrh2 gave some sources. Gary Imm's Compendium (search this Forum) can also be useful. A number of catalogue books may mention recommended combinations. But there's no one definitive source or tool. At least, to the best of my knowledge.


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#7 Sreesha

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Posted 05 February 2025 - 01:32 AM

Thank you this helps.

 

 

 

If there's Sii signal, then your image will benefit from having it because it will add extra details. The problem is that Sii is usually very weak, so it's a personal decision whether you want to include it or not, or how much time are you willing to spend on collecting the data.

 

These are the sources that I use to determine if there's enough Sii signal to make it worthwhile:

https://www.galactic...er-expectations

https://cosgrovescosmos.com/

They have mono images of SHO data normalized to show their relative strength for many different popular DSOs.

 

Here's a pretty good example (with strong-ish Sii) that demonstrates the effect of adding Sii to basically an RGB image:

51995229302_bff7481fd2_z.jpg 53321562137_1547f394a7.jpg

 

Ignore the blue colors in the second image - they should be in the first image as well, but this Oiii signal is very very weak, so it was not accessible to me at the time.

You see uniform red color of Ha in the first HOO image. But the second SHO image has a mix of the green of Ha and red of Sii, so it creates those red hues at the bottom on top of the green-yellow Ha signal. It's your preference which one you like better.



#8 bobzeq25

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Posted 05 February 2025 - 01:48 AM

To answer the basic question first. Sulfur usually has little contribution. You usually need to distort the data a lot to get it to have any. The more you distort the data the better you can see S. Whether or not it contributes to your images depends on both how much S is present and on how much you distort the data. In some cases it may border on impossible to get it to have any contribution. It's rarely essential. This is explained in more detail below.

A lot has been said here in confusing language, easily misinterpreted. Let me try to give a simple version.

Emission nebulae are much like neon signs. Hot gas emits light. Different elements emit different light. The light depends on the state of ionization of the elements.

IMPORTANT. The light is emitted in VERY narrow bands. Filters try to isolate these, but they can only do so approximately, their "bandpass" is significantly wider than the light emitted.

Also important. Elements generally emit light in several bands, as a practical matter one band tends to me much stronger than the rest.

Hydrogen emits light mostly at a wavelength of 656 nanometers (our eyes see it as red). An Halpha filter captures that.

Oxygen (triply ionized) emits light mostly at 501 nanometers (our eyes see it as blue/green). An O(III) filter captures that.

Sulfur (doubly ionized) emits light mostly at 672 nanometers. Our eyes have little sensitivity there, to the extent they can see it, it's red.

Emission nebulae generally emit those three lines, but the emissions are usually VERY different in intensity. Hydrogen usually dominates. Oxygen is usually less. Sulfur is even less, and it may be VERY weak.

And our eyes would not separate hydrogen and sulfur at all well. Sulfur can (if processed in a certain way) can add some color, but would essentially never add more shape to the hydrogen data because it almost always just sits on top of the hydrogen data.

In order to get all three visible to our eyes separately, people often take the signals from the 3 filters and change them to new colors, which often bear no resemblance to reality. Further unreal is that they VERY often artificially boost the three signals differently. If they did not do so, hydrogen would dominate, oxygen might get lost, and sulfur would usually get lost.

The famous "Pillars of Creation" image used all these techniques. Signals from the 3 filters were changed to three different colors. Oxygen was significantly boosted. Sulfur was boosted a LOT.

The whole business is where the business of imaging involves a LOT of art.

Some bottom lines. Sulfur can usually be omitted, to include it would require boosting it a LOT, and almost always it would add color but no additional detail, because it sits exactly on top of hydrogen. Very unrealistic. But some like the artistic effect.

Black and white hydrogen images are a great way to image nebulae, they capture the important light, and the result is quite realistic. Example below, click on it for a better version, and all the details. Note that it was done from a VERY light polluted backyard (Bortle 7) and it captured a great deal of very dim stuff.

I don't even own an S(II) filter, in my opinion there's no need for it. Almost always, it just adds a bit of color. No detail.

If you really want to understand all this, this is a fabulous book. Among other things, it talks about how professional astronomers measure a lot more than H, O, and S.
https://www.amazon.c...r/dp/1602232733


get.jpg?insecure

Edited by bobzeq25, 05 February 2025 - 02:39 AM.

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#9 licho52

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Posted 05 February 2025 - 04:28 AM

Practically speaking, SII is very situational and pretty much only used for false color. If you want to preserve more natural palette, SII is not that useful.

 

If you want to do SHO imaging then it's necessary however. Among amateurs, this kind of imaging has fallen out of favor as of late due to the explosion of remote imaging.



#10 imtl

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Posted 05 February 2025 - 04:48 AM

The goal in narrowband imaging in general and Sii specifically is not to map a color, but rather to capture and map different structures od emission nebulae.
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#11 DeepSky Di

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Posted 05 February 2025 - 08:30 AM

Hi Sreesha - SHO imaging is my happy place.

 

First off - https://www.galactic...er-expectations - this page will show you many examples of what to expect in H, S and O filters.

 

Taking the example of the Seagull Nebula from, this month's Goofi  challenge. I would put it like this: Ha covers the entire Seagull and has a lot of structure; Sii is like the wing bones and skull of the Seagull. Oiii is often not very structured and can be in a distinct area from the Ha or completely overlapped by the Ha.

 

The purpose of SHO imaging, for me, is to capture what's there - separating and visualizing the three different gas clouds, despite their overlapping, despite the Ha often drowning out the other two. We do this by assigning them 3 different RGB colors - normally Sii = red, Ha = green and Oiii = blue. At this point the picture would be all green. Removing the green from the picture is, in my opinion, not the right way to go. Instead, balancing the 3 channels with Linear Fit keeps all the data and allows a colorful image to result. And in general I'm aiming for something that people will want to hang on a wall, so it has to be bright and thought provoking. I am also experimenting with other color mappings such as HSO.

 

 

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  • Elephants Trunk SHO.jpeg

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#12 siovene

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Posted 05 February 2025 - 12:25 PM

Just to add to the discussion, since others mentioned AstroBin, you can search for images acquired using an SII filter (amongst dozens other search filter types):

 

Screenshot 2025-02-05 at 18.23.52.jpg


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#13 AstroVagabond

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Posted 05 February 2025 - 12:27 PM

 

Among amateurs, this kind of imaging has fallen out of favor as of late due to the explosion of remote imaging.

Can you expand more on your thoughts behind this comment? I'm neither agreeing or disagreeing just looking for additional clarification because I find it an interesting comment with a reference to remote imaging,

 

Thanks!



#14 Sreesha

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Posted 05 February 2025 - 12:50 PM

Just to add to the discussion, since others mentioned AstroBin, you can search for images acquired using an SII filter (amongst dozens other search filter types):

 

attachicon.gif Screenshot 2025-02-05 at 18.23.52.jpg

Understand and appreciate that.  However just because someone used Sii filter, it doesn't mean the subject has Sii data.  That is where the differentiation comes in which I am trying to seek.



#15 bobzeq25

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Posted Yesterday, 11:39 AM

Understand and appreciate that.  However just because someone used Sii filter, it doesn't mean the subject has Sii data.  That is where the differentiation comes in which I am trying to seek.

Not easy information to get. You could look for the S(II) color in those images. But, you have no way of knowing if it's due to a very skillful imager pulling out a very weak S(II) signal.

Here's what Gemini AI says.

"It's tricky to give you a definitive list of emission nebulae with the strongest S(II) signal, as "strong" is subjective and depends on factors like:

Imaging equipment: Filters used, camera sensitivity, etc., all play a role.
Processing: How the image is processed can enhance or diminish the S(II) signal.
Specific area of the nebula: S(II) might be strong in some parts and weaker in others.

However, some emission nebulae are known for having a significant S(II) component, and are often targeted by astrophotographers specifically for that reason. Here are a few examples:

NGC 1499 (California Nebula): This nebula is known for its strong S(II) signal, which contributes to its distinct appearance in narrowband images.

Sh2-155 (Cave Nebula): This nebula is another good example with a noticeable S(II) presence, often showing interesting structures in that channel.

Several supernova remnants: Supernova remnants, like the Veil Nebula or the Crab Nebula, often exhibit strong S(II) emission due to the shockwaves and interactions with surrounding gas."

I note in passing that it took Gemini a few seconds to give me that information. Beats spending hours looking for it.

We're in trouble. <smile>

Edited by bobzeq25, Yesterday, 11:44 AM.


#16 peterc

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Posted Yesterday, 01:26 PM

I'll add a link that's pretty good for this. https://www.galactic...er-expectations

 

From last night - I did a session on M42 since I'd only get a short session in.

The attached has the three channels done with the same basic EZ Soft Stretch at 15% and a histogram correction to set the black point.
(SII to the right, Ha to the left, OIII below).

Attached Thumbnails

  • M42_SHO_SII.jpg

Edited by peterc, Yesterday, 01:27 PM.



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