Noise reduction is done multiple times throughout the image processing project. Each noise reduction process focuses on one small aspect of noise.

Tip: It is best to have a light touch with noise reduction or else the image will look over processed.

Tip: Install the DeLinear script from Herbert Walter Astrophotography (skypixels.at) In the linear processing stage, we often need to create a clone then apply a permanent histogram stretch. The Delinear script does the stretch automatically.

Mask Creation

We need to create a mask so we can focus our noise reduction process to the small pixel noise in our images. To create the mask, we do the following:

  • Duplicate the image
  • Apply the DeLinear script – this does a permanent stretch to the cloned image
  • Launch Curves Transformation and apply the following to the cloned image
    • Point 1: .25
    • Point 2: .50
  • Launch Histogram Transformation and move the middle slider so the peak of the cloned image is at 50%

With the mask created, we need to apply it inverted to our image so we protect the signal portions and focus our noise reduction onto the low-level background.

Select Mask -> Show Mask to remove the red coloring

TGV Denoise

With the mask created and applied (inverted), we need to focus on our noise reduction parameters

It is often useful to create preview windows to test settings. Applying to a preview window makes the process occur much faster than if you applied to the full image.

For TGV Denoise, I use the following:

Because we are dealing with monochrome, we want to use the CIE L*a*B* mode and focus on the lightness.

After testing, I found out that I had to increase the exponent factor for edge protection to -4 from the default of -5.

I also increase the number of iterations to 500

Before
Before
After
After

Once the settings are identified, apply it to the entire image. This will take some time

Before TGV Denoise (HA Filter)
Before TGV Denoise (Green Filter)
After TGV Denoise (HA Filter)
After TGV Denoise (Green Filter)

Follow this process for all images (TGV Denoise settings will be the same)

What’s Next

The next step is to do another round noise reduction focusing on medium-level noise with the PixInsight Multiscale Linear Transform tool.