Removing Background Mottle with PixInsight

[STEVE333]

The following shows how to use PixelMath to remove MOTTLE from an image. Unfortunately I don't remember who came up with this PixelMath approach, but, it wasn't me. Thanks to the unknown contributor.

1) Open PixelMath and make sure the Use a single RGB/Kexpression is UNCHECKED. Enter the expression shown below into the R/K tab. Make sure that Generate output and Replace target image are both selected as shown.

2) Enter the expression shown below into the G tab. Note that the number inside the "[ ]" has changed to 1.

3) Enter the expression shown below into the B tab. Note that the number inside the "[ ]" has changed to 2.

4) Enter the constants a and b in the Symbols tab as shown in the image below.

5) Determine the values for a & b using the guidance provided below:

• Set the value for b slightly higher than the brightest mottle you wish to reduce. I determine this value by watching the preview while moving the cursor around the image with the left-click on the mouse depressed. As b is increased, more mottle will be reduced. However, raising b too high will start removing low level nebulosity.

• a controls how aggressively the mottle is reduced. Larger values of a will reduce mottle more.

• When a & b have been set then drag the PixelMath triangle onto the target image. If happy with the result you are done. If not happy, adjust a &/or b and try again.

An icon to perform the De-Mottle can be downloaded at the link below:

https://www.dropbox.com/s/osqh836d2940r ... .xpsm?dl=0

Hope this helps.

Steve

[Juno16]

Many thanks for the detailed and descriptive instructions Steve. I have saved the process icon. Thanks for that.

Is there a particular stage of the process this procedure works best? The final xisf?

[STEVE333]

Many thanks for the detailed and descriptive instructions Steve. I have saved the process icon. Thanks for that.

Is there a particular stage of the process this procedure works best? The final xisf?

Jim - I usually apply it the final stage, but, it could be applied to any stretched image. I often use it in conjunction with a mask to protect the target (nebula in my case).

Good luck,

Steve

[Juno16]

Many thanks for the detailed and descriptive instructions Steve. I have saved the process icon. Thanks for that.

Is there a particular stage of the process this procedure works best? The final xisf?

Jim - I usually apply it the final stage, but, it could be applied to any stretched image. I often use it in conjunction with a mask to protect the target (nebula in my case).

Good luck,

Steve

Absolutely. Makes perfect sense to use a mask to apply only to the background.
Sounds very basic, easy, and reversible if need be.
Thanks Steve for taking the time to very nicely outline this process.

[STEVE333]

One additional point. The icon I provided is setup to work on an RGB image. If you want to apply it to a monochrome image (like a Luminance image) just make sure the "Use a single RGB/K expression" box is checked as shown below.

Steve