NBRGBCombination script versus SHO/HOO+RGB Star Mask - Forums

Topic starter

Feb 6, 2022 · Edited: Feb 6, 2022

I think I've being combining my narrowband and RGB data in the wrong way and wondered what other people do. Up until now I've been using the NBRGBCombination script in PixInsight to combine the channels. Given that I'm mainly using the RGB data to keep my stars looking natural I wondered if the following may be a better technique:

1) Combine my RGB channels (pre-processed and colour calibrated) into a new RGB image using channel combination
2) Stretch that RGB image until the stars look how I would like them in the final image
3) Use Starnet to extract an RGB star mask
4) Do a medium stretch on my narrowband images and use starnet to remove the stars
5) Combine my narrowband images (SHO or HOO combination using ChannelCombination or some other ratio using Pixelmath)
6) Once happy with the final SHO/HOO image add the RGB star mask back in using Pixelmath

Any thoughts?

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andrea tasselli

Frequent contributor

Feb 6, 2022

Try it out and see how it looks. I'd never ever do it but that's just me.

Andy Wray

Topic starter

Feb 6, 2022

andrea tasselli:
I'd never ever do it but that's just me.

Any reason why? i.e. is my wish to get the star colour that way fundamentally flawed?

andrea tasselli

Frequent contributor

Feb 6, 2022

Apart from being questionable from my point of view, no. Assuming you do it properly, that is you get the field stars to merge smoothly against the NB background which might not be that easy in circumstances (e.g., halos and diffraction spikes).

Mike Cranfield

Feb 6, 2022 · Edited: Feb 6, 2022

The way you describe is the way I have done it and it seems to work OK but I am certainly no expert!

The biggest challenge I find is the final step - adding the RGB stars back in. I have seen people suggesting a simple PixelMath addition ie SHOImage + RGBStars. That can leave some areas clipped at 1 and can look a bit unnatural I have found.

I have tried using SHOImage + RGBStars - (SHOImage * RGBStars) - essentially a "screen" blend mode. This avoids the blow out (ie always returns a value in the range [0,1]) and seems to give a better/more natural result to my eye, at least on the images I have used it on. I would welcome thoughts from others on the best way to recombine the stars.

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Andy Wray

Topic starter

Feb 6, 2022 · Edited: Feb 6, 2022

andrea tasselli:
field stars to merge smoothly against the NB background which might not be that easy in circumstances

I see what you mean. Here's a really quick go at the method above and I ended up with this ... not a perfect merge as you say. That said, with the improvements in Starnet v 2 and spending more than ten minutes knocking this merge up, I think that it could work well:

MARK Shelton

Feb 16, 2022 · Edited: Feb 16, 2022

I find the NBRGBCombination tool extremely usable. I like the ability of the tool to experiment with numerous tweaks as you review the output.

I use an RGB image as a scaffold to add the narrow band images which have the stars removed so effectively you only add the narrow band data of interest to the RGB image less the stars.

I use the excellent starXTerminator- it is must faster and gives better results than Starnet and also works on linear data which is a big plus as well as coming out of the Russel Cronin stable of tools. However I've not looked at Starnet version 2 so it may be better or different.

As for adding the stars back into an image I use Pixelmath and give the tool no special data other than the RGB star target. It seems to work okay and you get a level of control by deciding if you rescale or not. You can always take your star image and maniplulate it prior to putting back into your final image- a slight blur can deal with any ragged edges left from star removal processes.

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Dominik Weinbrenner

Feb 16, 2022

I use the NBRGBCombination script only for boosting an actual RGB image with additional narrowband data.
However, if I'm going for a full (false color) SHO/HOO style composition, I do not use an RGB image as a basis. I compose the SHO/HOO image via LRGBCombination or PixelMath and add the RGB image using a star mask. That way I only get the RGB color where I want it - in the stars.

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MARK Shelton

Feb 16, 2022

Dominik Weinbrenner:
I use the NBRGBCombination script only for boosting an actual RGB image with additional narrowband data.
However, if I'm going for a full (false color) SHO/HOO style composition, I do not use an RGB image as a basis. I compose the SHO/HOO image via LRGBCombination or PixelMath and add the RGB image using a star mask. That way I only get the RGB color where I want it - in the stars.

I've used the masking technique myself to apply RGB colour to stars however I found that complete star removal, at least for me, seems to work better- however my PI "Kung-FU" is not yet that great so always room to try new things.

I've just given Starnet2 a test and I'm impressed with the results- it certainly will give StarXTerminator a run for its money and it does not cost a fee either- so good to have both in your tool kit.

Dominik Weinbrenner

Feb 16, 2022

MARK Shelton:
Dominik Weinbrenner:
I use the NBRGBCombination script only for boosting an actual RGB image with additional narrowband data.
However, if I'm going for a full (false color) SHO/HOO style composition, I do not use an RGB image as a basis. I compose the SHO/HOO image via LRGBCombination or PixelMath and add the RGB image using a star mask. That way I only get the RGB color where I want it - in the stars.

I've used the masking technique myself to apply RGB colour to stars however I found that complete star removal, at least for me, seems to work better- however my PI "Kung-FU" is not yet that great so always room to try new things.

I've just given Starnet2 a test and I'm impressed with the results- it certainly will give StarXTerminator a run for its money and it does not cost a fee either- so good to have both in your tool kit.

Mark, of course the stars should be removed from the individual channels prior to working on the nebulosity.
However, in the end I do not bother removing RGB stars from the RGB master to add them into the NB image. I usually create a star mask from the tight Ha channel, apply it to the SHO master and add the entire RGB image to it. That way you only get the RGB added in places where the NB stars are located.

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