ATrois Wavelets/K-Sigma noise reduction in Pixinsight
I always wished that noise reduction could be applied at the linear stage, afterall why go to the bother of stretching all the noise only to then try and get rid of it? Better to remove the noise at the linear stage then it is not included in the stretched image at all!
Pixinsight of course, has the answer in the form of the K-Sigma noise reduction in the ATrois Wavelets function. Open up ATrois wavelets on an image that has only a screen transfer stretch applied (This stretches the displayed image but the underlying image is still in its linear form) and click the K-Sigma noise reduction and marvel at the dissapearance of the noise! You can also apply some sharpening by increasing the detail layer bias by small amounts. Here is a before and after image, it’s from a 30 second exposure of the Orion Nebula (The sample is a tiny portion of the full image).
Here is the setting you want to enable:
Noise Reduction at this level also solves the patchy noise reduction artefacts that NR functions like ACNDR produce, the image below has been stretched then had ACNDR applied, it’s quite tricky to show in a low res shot but hopefully it shows some very dark, faint blobs, which are not present in the images above when K-Sigma NR has been applied.