A popular technique for creating stunning narrowband images of celestial objects is to use the Hubble Palette, which uses narrowband filters to map specific wavelengths of light to different colors. While this method can produce striking images, it often results in a strong green cast due to the dominant H-alpha signal. In this guide, I will walk you through a series of steps to process your narrowband image, focusing on reducing the green cast and enhancing the overall visual appeal. By using AutoIntegrate.js, masking, SCNR, Curves adjustment, and HistogramTransformation, you will be able to create a final image that accurately represents the vibrant colors captured by the Hubble Palette technique.

Initial Processing of Your Narrowband Image

Use AutoIntegrate.js at its default settings – just add your HSO files to AutoIntegrate.js and then click the Run button to execute the script. When the script completes, it will leave an open window called AutoRGB on your desktop, along with several minimized windows. One of the minimized windows is called Integration_RGB and we’ll use this file as our starting point.

Reducing the Green Background

Use the following steps to reduce the green cast in your image:

  1. Un-minimize the Integration_RGB image so that you can see it on your PixInsight desktop. The contents of the window will be very dark because the image has not been stretched.
  2. Use the ScreenTransferFunction to stretch your image automatically
  3. Transfer the STF to your HistogramTransformation process to make the stretch permanent.

    The image might have a significant green cast, and we’ll correct that next.
  4. With the Integration_RGB window active, from the menu, select Image – Extract – Ligntness to extract the luminance image from the RGB image.
  5. With the luminance window active, open HistogramTransformation, and, using the real-time preview, adjust the black point so that your image has a high degree of contrast.

    The idea behind this step is to enhance the high-signal parts of your image so that they are bright white and the low-signal parts of your image so that they are very dark – you should have significant differences between the dark and light parts of your image.

  6. Apply the modified luminance image as a mask to your stretched Integration_RGB image
  7. Invert the mask to protect the areas of your image having high signal
  8. Use the SCNR process at its default settings to remove the green cast from your image’s background
  9. Remove the mask from your image
  10. Start the HistogramTransformtaion process and, with the real-time preview open, click the Auto clip shadows button next to the Shadows area of the HT window to make the background darker
  11. Apply the HT process to the Integration_RGB image
  12. Open the Curves process and, with the Integration_RGB image active, activate the real-time preview
  13. Click the Hue button in the Curves window to activate the Hue curve
  14. Place four control points on the graph as shown in the following screenshot:


      • Move the second control point to the point shown in the following screenshot:



        The goal is to move the higher part of the green, shown along the bottom of the graph, to yellow, shown on the left side.

      • Adjust the first control point a little to smoothen the transition between the red and yellow
      • Move the third control point to the point shown in the following screenshot:



        The goal is to move the high-end green color over to the cyan color.

      • Adjust the fourth control point to make the transition between the red and blue smoother – this might be a very small adjustment
      • Evaluate the result in the real-time preview and adjust the second and third control points to minimize or eliminate the green in your image
      • Close the real-time preview and apply the Curves process to your image

      You now have your final image; adjust the saturation to taste and save your image.

      Conclusion

      The Hubble Palette technique often results in images with a strong green cast due to the dominant H-alpha signal. However, by following a series of steps involving AutoIntegrate.js, masking, SCNR, Curves adjustment, and HistogramTransformation, you can effectively reduce the green cast in your Hubble Palette image. This approach allows you to create a more visually appealing and representative final image, showcasing the vibrant colors captured by the Hubble Palette technique.