The Generalized Hyperbolic Stretch process and script are very powerful additions to the suite of tools offered on the PixInsight platform. In this article, I demonstrate how to balance the histogram using Eric Cole’s method, as described by Paulyman Astro in this YouTube video:

https://www.youtube.com/watch?v=upRoHNueEig

The linked YouTube video demonstrates the Eric Cole Histogram Balancing technique using the GHS script 2.0 and in this article, I demonstrate the updated and revised technique using the Generalized Hyperbolic Stretch process.

Result of Eric Cole’s Histogram Balancing Using GHS

The following image is a narrowband image of NGC 6188, the Fighting Dragons of Ara, processed using the SHO color palette and having the histogram balanced using Eric Cole’s method:

The image is still relatively green because the green channel is quite strong; however, there are subtle shades of red and blue in the nebula that might not be visible in the resolution-reduced version posted here. The colors are very subtle, but they are there, and you can perform further processing to highlight them.

For comparison, the following is the same image stretched based on PixInsight’s STF:

The image has a strong green cast, and the nebula is over-exposed, losing some details in the darker parts of the nebula.

Sample Download

I have made the combined SHO version of NGC 6188 available for download with this article so that you can follow along as I demonstrate Eric Cole’s histogram balancing technique using the Generalized Hyperbolic Stretch process in PixInsight. The sample download is a FITS file in its linear state and is ready for further processing.

Install the GHS Script and Process

If you haven’t already installed the Generalized Hyperbolic Stretch script and process, follow the directions here.

Eric Cole’s Histogram Balancing Technique using the GHS Process

I assume that you have the Generalized Hyperbolic Process installed within PixInsight and that you are somewhat familiar with using PixInsight: you know how to apply a process to an image, you know how to use a real-time preview, and you’re comfortable manipulating parameter values using sliders and numeric entry fields.

In this tutorial, we’ll do the following:

  • Balance the red and blue parts of the histogram using Eric Cole’s technique
  • Adjust the image’s lightness component
  • Clip the shadows

We’ll do all of this using the Generalized Hyperbolic Stretch process.

Begin by opening the FITS observation of NGC 6188. If you’re using your own image, process it to the point where you’re ready to stretch it and this might include having to use ABE or DBE on your image.

Do the following:

  1. Start the Generalized Hyperbolic Stretch process and reset it to ensure that it is in its default state
  2. Activate your image to ensure it is the image GHS is currently manipulating, and disable the STF by pressing CTRL+F12 on your keyboard or disabling it in the process window.
  3. Click the Big zoom in button once (the magnifying glass icon).

    This zooms in on the interesting part of this histogram so that it displays a closeup view. This is what my screen looks like:



    The histogram shows that green is the predominant color, followed by red and blue. The red and blue parts of the histogram are weaker than the green, as shown by the peak of the red and blue graph compared with the location of the peak of the green part of the graph. The goal is to move the red and blue parts of the graph over the green part so that the resulting histogram looks something like this:



    The peaks of the histogram are more closely aligned, resulting in a better image.
  4. In the GHS window, expand the Color Options area and select Red for the mode.
  5. Click to the left of the point on the histogram where the red begins to rise from the x-axis; this is what my screen looks like:



  6. Click the Send to SP button to set the Symmetry Point (SP) parameter.
  7. Click the Zoom 1:1 button to zoom out of the histogram view.
  8. Drag the Stretch factor slider until you see the red peak touching the first quarter of the graph; this is what my screen looks like:



    This action brightens the red part of the image. You don’t have to be precise with the placement of the red region on the graph.
  9. Click the Big zoom in button again to zoom in on the histogram.
  10. Copy the value in the Symmetry point field and paste it into the Protect highlights field.
  11. Manipulate the value in the Protect highlights field until you see the red peak at the location shown in the following screenshot:



    The idea is to align the red with the green. You may have to edit the value in the Protect highlights field using the individual numbers – experiment to find what works best.

    In my case, the number I found was 0.01, and I did that by pressing the up and down arrow keys on my keyboard while the cursor was in the Protect highlights field.
  12. Apply the stretch to your image by clicking the Apply button or by pressing F5 on your keyboard

    The image won’t change because we’re manipulating it in the linear space, and the change is very small at this point.
  13. Reset the GHS process.
  14. In the Color options section of GHS, select the Blue mode option.
  15. Repeat steps five to 12; the number I used for the Protect highlights field was 0.0105; this is what my screen looks like:



  16. Reset the GHS process and click the Zoom 1:1 button.
  17. Activate the real-time preview.
  18. Enter 10.0 in the Local intensity field.
  19. Drag the Stretch factor slider to the right until the peaks on the histogram graph reach about the 25% mark; this is what my screen looks like:



  20. Close the real-time preview and apply the change to your image to update it.
  21. Open the real-time preview and reset the GHS process.
  22. In the Color Options section, select the Lightness mode.

    This mode affects the image’s luminance only, allowing you to make changes without having to extract the luminance component and use it as a mask.
  23. Drag the Stretch factor slider to make your image lighter; I used 0.58.
  24. Close the real-time preview and apply the change to your image.
  25. Reset the GHS process
  26. In the Transformation type, select Linear

    This changes the stretch type from Generalized Hyperbolic to Linear.
  27. Manipulate the Blackpoint parameter so that your image’s background becomes darker; the value I used was 0.20.
  28. Close the real-time preview and apply the change to your image.

You now have your final image that’s ready for further processing; you could use ColorSaturation or other processes to highlight the image’s colors.

Conclusion

This article has provided a tutorial on using Eric Cole’s method to balance the histogram of an SHO image. You have learned how to execute the necessary steps using the Generalized Hyperbolic Process and have used its parameters to manipulate the image and achieve a final result.