Luminance masks are used many times during the PixInsight image processing workflow. The objective of the luminance mask is to apply a greater strength mask to high signal areas and lower strength to low signal areas.
Luminance masks are helpful with initial noise reduction, color saturation and brightness adjustments. Unlike star masks, luminance masks are quite easy to create.
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.
Create a Luminance Image
Creating a luminance image to be used as the mask is dependent on where on what phase of the image processing workflow you are at.
If we are still early on in the workflow (before creating a color image), we simply right-click on the image and duplicate. We now have a luminance image.
If, on the other hand, we have already combined our frames to create a color image, we need to use the Channel Extraction process.
Tip: There is an icon in the top shortcut bar that does the channel extraction automatically. It is called Extract CIE L* component. It is simply a shortcut to run the channel extraction process.
Tip: It is often a good idea to set the image identifier name to this image to better avoid future confusion.
If we are still in the linear phase of the image processing workflow, we need to stretch the luminance mask.
If the script is not installed, we simply do the following
- Screen Transfer Function
- Do an automatic stretch (radioactive button)
- Drag the triangle to the bottom bar in the Histogram Transformation tool
- This applies the screen transfer function temporary stretch to the Histogram Transformation configuration
- Apply the stretch to the image.
We want the mask to depict stronger/weaker signal areas, but we don’t want it to be pixel-level detail. Our mask should be slightly blurred. We can easily achieve this blurring effect with Multiscale Linear Transform.
By disabling the first 2 layers, we will create a slight blurring effect in the luminance mask.
We don’t want to remove too many layers or else we will start to blur stars, causing them to bloat.
Once we apply these settings, we now have a luminance mask.
This next part is optional based on the overall objective of the mask, but there might be a need to run Histogram Transformation again to lower the brightness levels to help focus the attention on the bright areas and much less on the dim areas.
One of the most common things to do is auto clip the shadows, which lowers the overall brightness slightly.