PixInsight offers a couple of ways of combining R, G, and B images into a single RGB image for further processing. The easiest to use process for this is ChannelCombination (the other way is to use the PixelMath process, but that makes things unnecessarily complicated for RGB/broadband images).

Using the ChannelCombination process is straightforward and this article demonstrates how to use it.

How To Use ChannelCombination To Produce An RGB Image  

You need to have at least three images open on your PixInsight desktop: an R, G, and B image. These images should already be integrated (aligned and stacked) and should be in the linear (non-stretched) state. Ideally, you perform ChannelCombination early in your workflow.

Start the ChannelCombination process by selecting from the menu Process – ChannelManagement – ChannelCombination (or select it from the <All Processes> option in the Process menu).

At its default settings, as shown, it is ready to use to combine and create an RGB image:

The RGB colorspace is selected by default, and that’s what we want to use. The R, G, and B channels are selected in the Channels area indicating you wish to use an image for each.

Click the icon on the right side of the window for the R channel as shown:

A new window pops-up where you can select the red image – select the appropriate image and click Ok. If you used AutoIntegrate and have the Integration_x.xsif files open, PixInsight automatically selects the correct image because it ends in ‘_R’ in the filename.

Repeat the preceding process for the G and B images.

Once you have all of the filenames filled-in, click the Apply Global icon – the third from left at the bottom-left of the ChannelCombination window:

A new image window pops-up with your combined RGB image – press CTRL+A to temporarily stretch the image so that you can see it (or open the ScreenTransferFunction process and click the Auto Stretch button (the yellow and black button on the left)).

Rename the image from its default, to something like ‘RGB’, to remind yourself that this is an RGB image.

A Note About Color Balance

When you press CTRL+A on your keyboard, PixInsight calculates a temporary stretch of your image. The image you see could end up looking pretty bad, like this one:

The image has a significant red bias – the bias on your image might be blue or green. This bias is happening because the image’s background has not yet been neutralized and we have not yet color calibrated the image. You can however correct this now without running those processes.

When PixInsight calculates the Auto Stretch, it assumes that all of the channels in your image have the same weight – so the red channel has the same weight as the blue and green channels. In this case, clearly that’s not correct due to the image’s red bias.

To temporarily correct this, open the ScreenTransferFunction process (located under Process – Intensity Transformations – ScreenTransfterFunction) and note that the channels are linked, as shown in the following screenshot:

Click the ‘Link RGB Channels’ button (the chain icon shown in the screenshot) to disable the function, then click the Auto Stretch button (the yellow and black button) to recalculate the Auto Stretch – your image should look more reasonable now, perhaps something like this:

This image is more color balanced.

Note that the preferred way of displaying an image is to have the R, G, and B channels linked however, in this case, we won’t be able to do that until we use either BackgroundNeutralization with ColorCalibration, or PhotometricColorCalibration process. The AutoIntegrate script that you use as part of this processing workflow applies both the BackgroundNeutralzation and ColorCalibration processes to your image.

Conclusion

In this article, you learned about the ChannelCombination process and learned how to use it to create an RGB image from three monochrome images.

More Articles In This Series

This article is part of a whole series of articles about processing images using PixInsight. Get the index article here, which explains an entire workflow for processing an image using PixInsight along with several useful scripts that make processing a lot easier than processing manually.