Swansea Astronomical Society Blog

The tools available for colourising narrowband images in Pixinsight have expanded considerably over the past years. New scripts developed by enthusiasts have increased the options available to all image processors without the need to learn complex Pixel Math expressions.

A couple of the new scripts enable Pixinsight users to harness the power of layer blending that is not a part of Pixinsight’s core processes. To a certain extent this enables some of the most useful features of Photoshop and Gimp to be used natively in Pixinsight.

This means that we are not bound by the usual R:G:B colour space combination with which to blend Ha; OIII; and SII linear images.

Image Blend Script

The image blend script has been developed by Mike Cranfield in collaboration with Adam Block. The script is available at Mike's website www.cosmicphotons.com

The script repository can be downloaded and installed in the Pixinsight Update tool to download the scripts and keep them updated.

This script allows two images to be “blended” using the blending modes used in Photoshop. Only two images can be blended which is a limitation. However, the real time preview means that you can experiment without needing to commit to a final image.

I use the script to blend two non-linear images that have been processed and stretched in the normal manner. The choice of stretch depends on the data. It is not necessary to finalise a stretch as modifications can be made in the script.

For an example I have blended two images of the Tadpoles Nebula (IC410). Data was collected in Ha;OIII and SII with two hours of data per channel.

The images to be blended are an HOO image and an SII image. The HOO image was created using Channel Combination in the usual way and then further processed in Narrowband Normalisation. The SII Image was processed and then stretched with Histogram Transformation. Both images are starless. This is not mandatory if you don't use Starnet++ or StarXterminator.Image 

Click on an image to get a closer view.

Blend Interface

The interface is easy to use following the instructions provided within the script.◘Here i have blended the SII image with a yellow hue. As the SII closely corresponds to the red of the Ha, the result is an orange colour surrounding the blue oxygen centre.

Here is an example of the same image using the Colour Dodge blending mode

This mode brightens the base layer and increases midtones vibrance in the base layer.The Adjustment Parameters panel allows changes to the shadows; midtones and highlights in much the same way that the Levels controls in Photoshop work. Lowering the midtones brightens the image. The shadows adjustment brightens or darkens the background and faint areas and highlights the brightest pixels.

Overall the Script works well and it is an interesting and creative way of blending Narrowband Images. The image can be further processed in Curves Transformation to increase the contrast, and Local Histogram Equalization for selective sharpening of details in combination with masks.

Narrowband Colour Mapper Script

This is another script written by Mike Cranfield and is available at his website www.cosmicphotons.com The script is a collaboration between Mike and Adam Block.

Script Interface:

The script allows for multiple images to be colourised and blended together. The only Blend Mode available is Screen Blending. This is fine as this is the most used blending mode. The separate images are loaded as greyscale linear images after basic processing. Again I use starless images, but images with stars can be used. At the top there is a screen transfer function interface that allows the background target level to be set.

The script has Histogram Controls; Modification Controls, Colour Controls, Background Neutralisation,

Output Options and Preview Dimensions.

The script allows for each Narrowband channel to be mapped to a particular colour and saturation and then the brightness and contrast can be modified using the Histogram Controls. Background Neutralisation uses the same Pixel Math as SPCC and a Background Preview box can be specified within the preview box.

Using the Script

In this example I am blending three images of the Elephant Nebula in Ha; OIII; SII. The order that the images are loaded does not matter. The Real Time Preview updates after each process is applied.

The order of processing I use is as follows:

1.​ The images to be blended need to be open before the script is launched.

2.​ Click Add and choose one of the views from the drop down box.

3.​ Click Update.

4.​ Add more layers in the same manner.

5.​ Click on the first layer and click Edit.

6.​ Select a colour and saturation level for the selected layer.

7.​ Click Update.

8.​ Repeat for the other layers.

9.​ To edit a layer click Edit and modify the Histogram controls. In the midtones, smaller values brighted

and higher values darken. Click Update and move to the next layer.

10.​Any parameter can be modified by clicking Edit, make the changes and then click Update.

11.​Carry out the Background Neutralisation selecting an area of background as a preview.

This script is very useful for experimenting with various blends of the Narrowband Channels. All edits are non destructive until the image is outputted. Further processing is then carried out to taste.

Paul Howat​

​9th May 2026

posted by astroimager  # 3:35 PM 0 Comments