Thanks, Leon and Michael. Thanks for pointing it out. I must have over adjusted the color level with GIMP. The walking noise starts showing up when I adjusted the colour level for the 2nd / 3rd time. The more I adjusted, the more is the walking noise.
I then used what Glenn had suggested,
https://www.youtube.com/watch?v=hIhgS3bvOy0
I tried the script, OSC_Preprocessing, but failed. Not sure if there is some problem with my flat images.
21:29:35: MAD is null. Statistics cannot be computed.
21:29:35: Normalization failed. Check image 1 first.
21:29:35: Execution time: 406.74 ms.
21:29:35: Setting CWD (Current Working Directory) to 'D:\Astro\19022022 2 - Copy'
21:29:35: Script execution failed.
I ended up using the script, OSC_Preprocessing_WithoutFlat, did a Colour Calibration and an autostretch (
It is so powerful, 1st time to use ) and the image has no walking noise and looks a lot better.
Thank you.
Regards
Terry