When I first started integrating narrowband images with RGB, I used the PixInsight NBRGB Combination script. It is simply a script that does the work for me. Easy. But now, I’ve been using a different approach that I will call Narrowband Channel Combination. Here is an example of using NBRGB Combination and Narrowband Channel Combination.

Take a look at the three examples, one is without narrowband integration, the second is using the NBRGB Combination Script, and the final is this alternative method.

RGB
NBRGB Combination Script
Narrowband Channel Combination

For each of these images, I used the same stretch.

  • RGB: Very little color in the center of the galaxy
  • NBRGB Combination Script: The entire galaxy has a red hue. Through curves transformation, this hue can be reduced
  • Narrowband Channel Combination: The center of the galaxy takes on a nice red tint on the areas with stronger hydrogen-alpha signal (HA)

The following details how to do the Narrowband Channel Combination method.

Channel Extraction

With our R, G and B images combined into a single RGB image, the first step is to extract the Red channel with the Channel Extraction process.

Channel Extraction
Channel Extraction

Depending on the narrowband filter getting integrated determines what color channel to extract

  • HA: Extract red channel
  • OIII: Extract blue channel

Linear Fit

With the color channel extracted, we must align the brightness levels between the narrowband image and the R, B or G image. This is done with Linear Fit.

Linear Fit narrowband
Linear Fit Narrowband

For the reference image, select the brighter of the two images, usually the R, G or B image is brighter than the narrowband image. Apply this process to the darker image.

Pixel Math

To merge the narrowband image with the R, G or B image, we will use Pixel Math to blend the two images together with a Screen Blend mode.

Pixel Math - Screen Blend
Pixel Math – Screen Blend

The screen blend mode inverts each image, multiples the images together, then inverts the final product.

Screen blend lightens the overall image. Although the Lighten blend mode, Max(IMG1,IMG2), would also lighten the image, screen blend mode creates smoother transitions. Feel free to try both.

Red Channel
Red Channel
HA Channel
HA Channel
Blended (Screen Blend Mode)
Blended (Screen)

As you can see from the three examples, the screen blend mode brings out the details captured with the HA filter while maintaining the brightness of the rest of the image from the Red filter.

Linear Fit

With the Red and HA channels blended, we need to do another linear fit before creating our final RGB image. The screen blend mode will have brightened the overall image. If we try to combine this new channel with the previous green and blue channel, the red channel will be too strong.

Simply reapply Linear Fit to the new blended channel, selecting either the previously fitted green or blue channel as the reference.

Channel Combination

Use the channel combination process and replace the plain red channel with the HA enhanced red channel.

Channel Combination
Channel Combination

Once done, you end up with the following

RGB
HA+RGB

What’s Next

Based on my experiences, this process works better than the NBRGB Combination Script.

The next step is to run through some color calibration processes.

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