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Some questions on adding Ha to LRGB/RGB

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#1 jtrezzo

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Posted 14 November 2017 - 01:00 PM

So this is a bit of a hodgepodge of questions regarding Ha and LRGB combination that I've been struggling with, and just keep finding a bunch of related but not exact answers to my questions, so hoping to clarify things a bit here...I am just starting out with most of this mono stuff and the processing is quite a bit more challenging than OSC.

 

 

1. What is the best method for adding Ha to an already processed color image? For example I have some Ha data of M31 and an already processed M31 from my OSC and want to add it. Do I have to go back and start all processing from scratch or is there some way to add it in reasonably well without starting over? 

 

2. When adding Ha to LRGB (let's say from scratch this time) should you process the Ha individually first (i.e. noise reduction, curves, etc.) or just combine it with L and R channel at the start and then go about everything else normally? What is the best ratio of combination to use? I have been using PixelMath to say something like "L + 0.8*Ha" or "R + 0.5*Ha) - not sure if that is the best way to do it.

3. For emission nebula like the California Nebula, is there any benefit to doing HaLRGB, or is just HaRGB fine? I've been looking up images of it and I am finding a lot that are just HaRGB. I ask because I have a bunch of Ha on it and want to start adding color and not sure if I need to get Luminance. What benefit would it provide, if any? Are there any guidelines for when to use HaLRGB vs just HaRGB? 


Edited by jtrezzo, 14 November 2017 - 01:04 PM.


#2 KBALLZZ

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Posted 14 November 2017 - 01:44 PM

I’m still trying to figure out the best method myself, but I’ll share the techniques I’ve played with so far and try to answer your questions.

 

Methods to combine post-stretch:

1. Download the old Multichannel Synthesis scripts and play with the HaRVB-AIP script

2. Try the NBRGB Combination script with the “nonlinear” box checked, an RGB bandwidth of 200, Ha bandwidth set to your specific filter’s pass, and the scale parameter adjust to taste.

 

Methods to combine in linear state:

3. Same as #2 above, but don’t check the nonlinear parameter (before color calibration).

4. After background extraction of the R,G,and B channels, use Pixelmath to blend the Ha into them (I like starting at 50%Ha in the Red, 10% in the Green, and 10% in the Blue). Then channel combine and proceed to process as an LRGB image, using your Ha as L.

5. The method on the PI website which I haven’t tried yet (https://pixinsight.c...als/narrowband/)

 

I’ve actually been playing with my Crescent Nebula HaRGB data all morning trying to find a natural feeling color, and so far I really am liking combining in the linear state using method #4. Just play with the percentages until you have a nice light red tone, not too red and not too hot-pink.

 

In terms of when to use HaRGB vs HaLRGB I look at it this way:

If there are any galaxies you wish to show, reflection nebula, or non-emmision line molecular clouds, use HaLRGB.

If the target is 99% Ha (like California, Crescent, Horsehead), use HaRGB.

 

Im looking forward to other responses here as well, there are so many ways to go about this.


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#3 bobzeq25

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Posted 14 November 2017 - 02:46 PM

I also like the NBRGB script in PixInsight.  Some fairly sophisticated things are going on in the background.

 

Adding L to something like HaRGB in the California Nebula has a tendency to turn the red nebula salmon colored. 


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#4 Jon Rista

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Posted 14 November 2017 - 02:58 PM

First off, the vast majority of my experience is with PixInsight, however you can blend data in PS as well, it's just done differently. I'll use PI for examples. 
 

1. What is the best method for adding Ha to an already processed color image? For example I have some Ha data of M31 and an already processed M31 from my OSC and want to add it. Do I have to go back and start all processing from scratch or is there some way to add it in reasonably well without starting over?


It is possible to blend Ha directly into existing color channels. You can just to a ratio blend, maintaining a total of 1.0/100% for the whole channel. 

 

I usually use PixelMath for blending. In PS, you could combine channels separately in different images then paste those channels into the appropriate RGB channels in a new image. You could use layer blending to adjust the percentage of the blends.

 

In PixelMath, just multiply the color channel of the target image ($T in PM, and it's indexed, so 0 = red, 1 = green, 2 = blue) by the Ha channel (or, for that matter, any nb channel, or even multiple nb channels at once). So if you wanted to blend Ha into R by 70%, blue by 10%, and green by 2.5%, and OIII into B by 20% and G by 10%:

 

R: $T[0]*.3 + Ha*.7
G: $T[1]*.875 + Ha*.025 + OIII*.1
B: $T[2]*.7 + Ha*.1 * OIII*.2

 

This will produce an image with the desired blend, by combining an existing color image (RGB or LRGB) with both a separate Ha and OIII channel.
 

2. When adding Ha to LRGB (let's say from scratch this time) should you process the Ha individually first (i.e. noise reduction, curves, etc.) or just combine it with L and R channel at the start and then go about everything else normally? What is the best ratio of combination to use? I have been using PixelMath to say something like "L + 0.8*Ha" or "R + 0.5*Ha) - not sure if that is the best way to do it.


I would generally process all channels individually first, then combine as appropriate. You can combine in many ways. You may "pre-combine" the Ha with R (and maybe B, and even G!) before combining into RGB, then combine the L. You could do the LRGB combination first, then blend in Ha into the RGB channels after.

 

Blend as described above.

 

3. For emission nebula like the California Nebula, is there any benefit to doing HaLRGB, or is just HaRGB fine? I've been looking up images of it and I am finding a lot that are just HaRGB. I ask because I have a bunch of Ha on it and want to start adding color and not sure if I need to get Luminance. What benefit would it provide, if any? Are there any guidelines for when to use HaLRGB vs just HaRGB?

I would be wary of making Ha your L, or of blending Ha into L. The Ha signal is a very, very narrow emission, whereas L is the entire spectrum. The two will often, and usually do, represent very different structure. Blending a little Ha into L could work, but replacing L with Ha entirely will often result in some strange colors and strange structure, as the Ha dominates the detail and structure perceptions, and will often NOT match the full structure across all color channels. A great example of this that is easy to demonstrate is to try to use Ha as luminance on M42 (Orion Nebula). There is a good deal of blue and even green (which ultimately makes grayish color when combined into RGB) in that object (especially Running Man next to M42). When you use Ha as L, the whole object takes on a very strange appearance, and the detail clearly doesn't match the color. 

 

Ha is just one emission of excited hydrogen gas. The gas actually emits in several bands, the two most prominent being Ha and Hb. I think a better way to take advantage of Ha is to blend it into R and B, more into R and less into B. And you may even find that blending a small amount into G helps. The key is that you control the ratios at which it is blended in, so it's not full force in every channel. This can improve detail, as well as color. 


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#5 jtrezzo

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Posted 14 November 2017 - 06:27 PM

Thanks Jon, Bob, and Andrew. This is all really helpful information.

 

That Pixinsight method number 5 Andrew alludes to sounds very interesting, but it doesn't seem to offer much explanation as to how to actually go about doing it. Hmm....

Jon, thank you for those PM formulae, I will have to give those a try. Do you agree with Bob about the California Nebula or would you do L for that one too? I know your example of M42 makes sense, but I've never seen much more than just red hydrogen in the California nebula. 

 

Another question regarding blending the Ha into an already processed OSC image, just to make sure I have this right. This is going to be for my M31 image, if it matters. So I should process the Ha (I'd assume for maximum contrast) then extract the RGB channels from my processed OSC image, use PixelMath to blend the Ha channel in using those formulae posted above, then recombine to RGB. Does this involve extracting the L channel and doing any blending at all with that, or just ignore it? My thinking is it would not really matter since it is from OSC and there is no real Luminance and it would just be synthetic (but would this matter for an LRGB shot in mono?).


Edited by jtrezzo, 14 November 2017 - 06:27 PM.


#6 Jon Rista

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Posted 14 November 2017 - 07:46 PM

I would just blend the Ha into R, and a little into B. Since it's OSC, no need to worry about L. Even if you had LRGB, no reason you couldn't just blend the Ha into that the same way.



#7 KBALLZZ

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Posted 15 November 2017 - 03:04 PM

Hmm yeah using Ha as L doesn't work as great as I thought it would in terms of color. IMO, there is just so much detail and goodies to be brought out by processing the Ha by itself and blending in later. I guess I've had the best luck by blending the Ha into the color channels in the Linear state, and then processing the RGB and Ha fully, and then combining the processed Ha back in. That last step takes the most time to "look right", and the HaRVB-AIP script seems to work out nice for that workflow. Still searching for that perfect workflow though. Wish it was as easy as LRGB!


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#8 KBALLZZ

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Posted 15 November 2017 - 10:33 PM

Here is an edit I came up with tonight where I'm quite happy with the colors and detail.

Processing went something like:
After background extraction of the R,G,and B channels, used Pixelmath to blend the Ha into them (50%Ha in the Red, 10% in the Green, and 10% in the Blue).
Channel combine to make RGB
PCC and an SCNR
Histogram Stretch
Processed Ha heavily (decon, NR, stretched, exponential transformation to push nebulosity, LHE)
Combined them both using the HaRVB-AIP script (lightness value set to 0.6, all other parameters at default).
Final SCNR and a small midpoint transfer function.

NGC6888_CN.jpg


Edited by KBALLZZ, 15 November 2017 - 10:35 PM.

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#9 spersky

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Posted 16 November 2017 - 06:55 AM

I use this method in photoshop.  I am not sure if you requested Photoshop or Pixinsight method, but this method is very effective.

 

http://bf-astro.com/tutorial/addHa.htm




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