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