Nebula Color with OSC (ZWO ASI2600MC)

Jerry Gerber:
I notice that some nebula I shoot display multiple colors, including red, orange, blue and green, like these:

https://www.jerrygerber.com/photography/IC1396.jpg
https://www.jerrygerber.com/photography/SH-132.jpg
https://www.jerrygerber.com/photography/SH2-296.jpg

Other nebula only display red and orange:
https://www.jerrygerber.com/photography/NGC7380.jpg
https://www.jerrygerber.com/photography/IC5070.jpg

It doesn't seem to be dependent upon which filters I use as both the Optolong L-Enhance and the L-Ultimate display different colors for different nebula.
When I shot this nebula a few weeks ago, I didn't use any filter and got these colors:

https://www.jerrygerber.com/photography/IC1848.jpg

My question is why do some nebula seem to have a greater variety of color and some do not?  I obviously do not shoot in SHO as I am using an OSC.

Jerry

Probably because something is wrong.

andrea tasselli:

Jerry Gerber:
I notice that some nebula I shoot display multiple colors, including red, orange, blue and green, like these:

https://www.jerrygerber.com/photography/IC1396.jpg
https://www.jerrygerber.com/photography/SH-132.jpg
https://www.jerrygerber.com/photography/SH2-296.jpg

Other nebula only display red and orange:
https://www.jerrygerber.com/photography/NGC7380.jpg
https://www.jerrygerber.com/photography/IC5070.jpg

It doesn't seem to be dependent upon which filters I use as both the Optolong L-Enhance and the L-Ultimate display different colors for different nebula.
When I shot this nebula a few weeks ago, I didn't use any filter and got these colors:

https://www.jerrygerber.com/photography/IC1848.jpg

My question is why do some nebula seem to have a greater variety of color and some do not?  I obviously do not shoot in SHO as I am using an OSC.

Jerry

Probably because something is wrong.

Well that's incredibly insightful and helpful. Well done!

Hi Jerry, Emission nebula do vary in the amount of oxygen III emission (blue colour) they have relative to the dominant hydrogen alpha emission (red colour). The Sh 2-132 (Lion Nebula) does have a big patch of OIII that you have picked up (and you can see it in other people's photos). Because it is much dimmer than the Ha you will only bring it up with unlinked stretches (stretching the blue/green independently of the red). There are lots of ways of doing this and you can also combine the blue and green channels to create an OIII specific channel - Cuiv the Lazy Geek and  many others have tutorials on this. The two filters you mention both pass light at these two frequencies and just differ in how narrow the bands they pass is (i.e., how much they remove other frequencies which will be light pollution).  But essentially it is down to how much OIII there is in the nebula and how much you try to bring it up relative to the dominant red in processing. If you keep them with their same relative intensity you will only ever see red for most emission nebula (but some don't have much OIII so they may be red anyway).

Very nice images by the way - going to give the Lion Nebula a go next time I get a clear night, inspired by yours.

Álvaro Méndez:

andrea tasselli:

Jerry Gerber:
I notice that some nebula I shoot display multiple colors, including red, orange, blue and green, like these:

https://www.jerrygerber.com/photography/IC1396.jpg
https://www.jerrygerber.com/photography/SH-132.jpg
https://www.jerrygerber.com/photography/SH2-296.jpg

Other nebula only display red and orange:
https://www.jerrygerber.com/photography/NGC7380.jpg
https://www.jerrygerber.com/photography/IC5070.jpg

It doesn't seem to be dependent upon which filters I use as both the Optolong L-Enhance and the L-Ultimate display different colors for different nebula.
When I shot this nebula a few weeks ago, I didn't use any filter and got these colors:

https://www.jerrygerber.com/photography/IC1848.jpg

My question is why do some nebula seem to have a greater variety of color and some do not?  I obviously do not shoot in SHO as I am using an OSC.

Jerry

Probably because something is wrong.

Andrea, this is a community where people share and learn from each other. I personally find very sad when people like you only (and often) answer to show disdain to say the least. My advice in cases like these is simple: close your socials, it will make more sense and you’ll be happier.

*Well said! I wish I had chosen to answer with your eloquence rather than my sarcasm! 

And I belive you and the others are giving the correct explanation.

Jerry
I think I can add something to this discussion, I do use the narrow band filters L-eNhance and L-eXtreme)for nebula, so I do have some experience. 

As mentioned already, with these filters you are for the most part getting just OIII (L-eNhance contaminates this with a little Hb, which will mimic the Ha signal, but generally relatively faint). So areas rich in OIII only should appear teal (a balance of green and blue) and Ha dominant areas red. In the nebula that you show with no obvious teal color, but white or orange etc there is OIII but in the same location it is not as strong as the Ha in that area, so you get a shade that is not quite red. If there is no OIII the whole nebula will be red.

Changing filters changes the bandwidths and also changes the efficiency of the filters. For example, OIII signal is right where red and green filters overlap in the OSC camera. But generally green is picking the signal up with more efficiency than blue pixels, and there are two green to one blue, so green tends to be stronger. To get a nice teal color for strong OII regions green generally needs to be de-emphasized. But when you change filters you will change the balance of color between green and blue in OIII regions and between the green-blue with the Ha, since you change how much light each filter is taking in. These filters vary a lot, so trying to use the same color adjustments to get the same color with different filters just is not going to work. You will need to change your color calibration when you change your filter to get the same result–if that is what you want. More about color choice below.

But remember, if you don't know how much red from Ha is in an area of the image that also has OIII you don't a priori know what color the nebula should be. This makes it difficult to calibrate color. In fact, one can argue that correct color in NB is ill-defined. Since our eyes don't see with NB filters, what color is right?–our eyes are very sensitive to OIII actually, but not very sensitive at all to Ha. In either case the NB filter in no way really mimics what we would see. Calibration works for stars, because they are of defined color, as long as we use RGB filters only. Even for stars, with NB calibration can only get close to true color with some channel mixing, but even then it won't be exactly right. So for NB nebula, and especially in SHO where there is even more color choice, you do have a lot of choice on how you want it to look. In the Wizard for example you can bring out blue and green with the right stretches, or you can leave it with no obvious blue or green as you have. One might argue that the latter is more natural, but with a different filter than you used that blue/green might show up naturally. 

One final thing, NB filters do let in continuum light in the blue/green and red, the narrow the filter the less of this of course. So you may be picking up more than just OIII and Ha from the nebula. And you will be picking up some light pollution with these filters too. This can change the color. To get rid of the continuum contribution, and just get the OIII/Ha, you can subtract the continuum signal by subracting some fraction of a comparable RGB image. Not necessarily easy to do, and beyond this thread to explain (and I have not done it), so I will not say more about it.

Hope this helps
Rick

Jerry,  it very much depends on how you process the OSC image. It looks like you are processing in the same way as you would a wideband image, so you will get what you get and colours will be dependent on the relative proportions of Ha and Oiii in the nebula.

An alternative approach that you might want to experiment with is to create an Ha image and an Oiii image from your data and process them as a narrowband image.

You can split the integrated image into its R, G and B constituents, optionally after extracting the stars.

With the filters you have, Ha = Red and Oiii can be constructed as (0.7 * Green + 0.3 * Blue) (for example).

You can linear fit Oiii to Ha.

You can then run your noise processing on the Ha an Oiii and optionally on a luminance created from the original before splitting (or use Ha as luminance).

You can then stretch Ha, Oiii and L (if using) to similar levels and combine back to a colour image using one of various techniques.

My preference is HOO using a Power of Inverted Pixels equation in PixelMath which accentuates difference in relative concentrations of the gases:// R:Ha// G(Oiii*Ha)^~(Oiii*Ha))*Ha + ~((Oiii*Ha)^~(Oiii*Ha))*Oiii// Biii

An LRGB combination puts it back together again (the L image gets extra attention to bringing out detail and sharpness)You can then adjust the colour mix to your preference with a combination of Curves, ColorSaturation, ColorMask, etc. (if using PixInsight) 

Hope this helps…

Roger