Galaxy processing - adding Ha to RGB without losing nebulosity in PixInsight

[BrendanC]

Hi all,

While the weather in the UK continues to stink, I've decided to hone my PI processing skills (or lack thereof).

I never really got the hang of adding Ha from a OSC dualband image for nebulosity, to an OSC RGB image, especially with galaxies, and especially with M101.

What I tend to find is:

• When extracting the stars from the Ha image (using StarXTerminator now, but even back when I used StarNet), it  pulls out some of the nebulosity too, because it recognises them as stars, and I can't figure out how to get it back, or stop this from happening in the first place.
• When combining the Ha with the RGB, I get a red cast from the Ha across the entire image.

So I'm looking for ways to extract just the stars from the Ha image, while preserving all of the nebulosity (or, how to put it back in); and how best to combine the Ha starless with the RGB starless (and then I apply the RGB stars via PixelMath, which isn't a problem).

I've tried several methods, from extracting the channels and recombining the Ha as red, also using an HaRGB script I came across on the Visible Dark YT channel, also various masking techniques - again, including one from Visible Dark where you use the Ha image as a mask - but nothing quite seems right. I seem to get too much Ha, so it's all a wash of red or purple and loses that lovely blue hue; or not enough, where it's almost as if there's no pronounced Ha nebulosity in the image at all.

Does anyone know of a good workflow that will help with this? Or, ideally, a nice video that walks me through it?

Thanks
Brendan

Edited January 5 by BrendanC

[ollypenrice]

Use Photoshop...

lly

[saac]

@BrendanC you've probably already looked through the PixInsight resource section I guess but I wonder if this routine is of any help?  Other than that it may be worth while asking on the PixInsight forum, it's a really good knowledge base and somebody will certainly have the definitive answer.  

https://pixinsight.com.ar/en/processing-examples/mask-background-stars-35.html

PixInsight Community Forum

Jim 

Edited January 5 by saac

[wimvb]

I use this method with a few tweaks. One of them is to use a mask that covers everything but the galaxy. 

https://pixinsight.com/examples/M31-Ha/index.html#Continuum_Subtraction

For osc, I would

• Extract the channels
• Use pixelmath on red (R) and Ha: Ha - F×(R - med (R))  where F is a number lower than 1, usually 0.4 -0.6 for me. This creates a new image: Ha-R
• Add this new image back into the red with pixelmath and a mask: R + F×(Ha-R -med(Ha-R)), where F is a new number, depending on how strong you want the Ha to be.
• Combine the channels with this latest image going into the red channel.

Hope this helps

Edited January 10 by wimvb