At this point, it is time to permanently stretch our image into a non-linear state. As with most things in PixInsight, there are multiple ways to permanently stretch an image.
Note: Before we stretch the image, we should have extracted the luminance channel that we can enhance later. Ideally, the luminance extraction should have occurred immediately after integrating the different channels into a single image. If the luminance hasn’t been extracted, do it before continuing.
Histogram transformation was my initial go-to approach for stretching images to non-linear. It is easy and it is fast, but I’ve found out that it bloats my stars.
Although I no longer use this approach, I’m only putting it in here as a comparison.
In order to do a stretch with Histogram Transformation, you first use the Screen Transfer Function with the channels unlinked (chain deselected). Hitting the radioactive button will apply a temporary stretch to the image.
Dragging the triangle from the Screen Transfer Function to the bottom bar of the Histogram Transformation process, we can copy the stretch settings, so we can permanently apply them.
The Screen Transfer Function stretches too much, so I move the middle slider to the right to darken the image. I like to have the peaks in the middle of the 2nd set of boxes on the graph. This yields the following stretched image.
Overall, it looks really good and I was often happy with this approach, until I started using Masked Stretch.
Masked stretch is the approach I use most often because it is easy and yields good results without overstretching or blowing out stars.
This image will not be as bright as the temporary stretch done so far because the temporary stretch is too extreme as it overexposes the background. In steps that follow, we will selectively brighten areas of the image while leaving the background intact.
If we compare Masked Stretch with the Histogram Transformation Stretch, you can see how the stars in the Masked Stretch are smaller. And although the Histogram stretch is brighter, it also greatly increased the background brightness as well, which we want to avoid.
The next step is to further remove the green and magenta colors from our images with SCNR.