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.

With multiscale linear transform, we will focus on the larger noise within the image, while trying hard not to reduce the quality of our signal.

Mask Creation

We need to create a mask so we can focus our noise reduction process to the low signal areas of 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 Histogram Transformation and move the middle slider so the peak of the cloned image is at 75%

You will notice that the mask is very bright. This is the signal. We will apply this to our image inverted so we protect everything that has a signal.

As you an see, with the mask applied and inverted, most of the image is red. This is protected from our noise reduction process.

Select Mask -> Show Mask to remove the red coloring

Multiscale Linear Transform

Multiscale linear transform works with multiple layers in our image.

  • Layer 1: 1 pixel size
  • Layer 2: 2 pixel size
  • Layer 3: 4 pixel size
  • Layer 4: 8 pixel size
  • Layer 5: 16 pixel size
  • Layer 6: 32 pixel size
  • R: Everything else

Smaller layers often have more noise than the larger layers and can handle more intense noise reduction.

For my setup, I’ve found at this stage, the following configuration works for me:

I limit multiscale linear transform to 6 layers. Going higher does not have an impact on the noise reduction with so much of the image protected with the mask.

For each layer, we need to adjust the following:

  • Threshold: This is essentially how strong to do noise reduction on the layer
  • Amount: For the final image, what blending percentage should be from the new image and old image. A value of 1 means there is no blending with the old image. With .60, 60% is new image and 40% is old image.
  • Iterations: How many times should this run on this layer.

With multiple layers, we want to decrease the strength as our layers increase in size. This helps protect the structure.

This setting yields the following.

Before MLT (HA Filter)
Before MLT (Red Filter)

After MLT (HA Filter)
After MLT (Red Filter)

The changes are very subtle. It is easier to see when zoomed in

Before MLT (HA Filter)
Before MLT (Red Filter)
After MLT (HA Filter)
After MLT (Red Filter)

Follow this process for all images (MLT settings will be the same) but create a unique mask for each filter.

Tips

For channels with low levels of signal, they will most likely have a lot of noise. It is oftentimes worthwhile to apply noise reduction multiple times.

What’s Next

The next step is to try an increase the clarity and sharpness in our high-signal areas with PixInsight Deconvolution.