Eliminating noise around stars in separated star images (using Pixinsight)

Programs and plugins like Starnet++ and StarXTerminator make it possible to separate stars from the nebulous regions in astro images. This allows for easier and more precise editing of the nebula before eventually recombining with the stars at the end of post processing. It has become a very common method of editing astro images.

One of the problems I've had when using this method is the amount of noise that is also separated with the stars. In the image below, I used StarXTerminator to remove the stars - which works very well, but inevitably leaves a bit of noise around the stars:

This is a tight crop. Exposure increased to highlight noise.

If we fully process the nebula then recombine it with the "noisy" star image, we get a result like this when we zoom in. Note the noisy "rings" around the stars:

So how do we solve this problem? I've experimented with a few solutions (including other software programs), but the best way I have found is by using TGV Denoise in Pixinsight. Here is a link to download my saved icon file containing settings that I find to work very well with most images (drag and drop it into Pixinsight).

When working with star-only images, I use TGV Denoise in CIE L*a*b* mode so that lightness and chrominance have separate control parameters. Here are the lightness settings:

And here are the chrominance settings:

You'll notice the chrominance settings are more aggressive than lightness. I prefer this because it incidentally helps with color fringing in stars (CA). This allows for further saturation of the stars without amplifying the fringing.

Here are the before and after results when applying the settings saved to the icon file (tight cropped image, stretched a bit more to emphasize difference):

An excellent result, in my opinion. The stars don't appear to be over-blurred or lose their sharpness. Of course, there are ways to apply deconvolution to the non-linear image if you'd like to tighten them up a bit (I'll save that for a future blog post).

Here is the result when the noise-reduced stars are combined with the nebula (tight crop). You'll see the noise is all but eliminated from the areas surrounding the stars, and they blend nicely:

Every star image will have differing results. What I've found works best is to modify the "edge protection" slider in the lightness controls to adapt to different fields/levels of noise. For example, if you have a wider FOV with many more stars, try bumping edge protection to about 5. You must be careful, though - if you are too aggressive, faint stars will begin disappearing.

The image used for this post is IC 405 "The Flaming Star Nebula", which was shot using my Celestron Edge HD 8 and the ASI294MM (SHO palette). You can see the full image here.

Thanks for checking out this post. Please feel free to comment with other solutions or feedback! Clear skies-

Chad