I am sure i read a few weeks ago the Juan recommended using a sharpening routine paired with the NR to reduce it
I am not sure if you refer to MultiscaleLinearTransform (MLT) or MultiscaleMedianTransform (MMT). If it's MMT, I assume you refer to this tutorial:
http://pixinsight.com/tutorials/mmt-noise-reduction/In this case those dark artifacts are the result of not removing outlier structures correctly with MMT. You have to use the
threshold and
adaptive noise reduction parameters to reduce noise and prevent these outliers on each transform layer. These parameters are described in the tutorial. Excerpting the most relevant paragraphs:
- Threshold. Increase the threshold parameter to remove more image structures. Typically, a value around 3 or 4 MAD is appropriate for the first layer. Successive layers normally require smaller thresholds (the larger the layer scale, the smaller threshold is usually required), but the actual values depend on the image and its noise distribution. Too large of a threshold will remove significant structures. Try to adjust this parameter to the smaller value able to remove the noise as you wish on each layer.
- Adaptive. Increase this parameter when you see isolated, high-contrast, relatively small structures that survive after finding an otherwise good noise threshold value. As happens with the threshold parameter, the less is the better: always try to find the smallest adaptive noise parameter that can remove the unwanted noise structures.
If you are using MLT, then you are not applying a correct noise reduction process with the same result: false structures generated by noise that escapes your noise reduction. MLT can be more difficult to control than MMT in this sense, mainly because MMT is much better at isolating structures. However, MLT can be better to reduce heavy background noise. Try to adjust the
threshold parameter on each MLT layer to prevent these artifacts. If necessary, play with the
amount and
iterations parameters to fine tune your noise reduction. With these images, always use a linear mask with both MLT and MMT. This will allow you to apply a much stronger noise reduction. See this processing example:
http://pixinsight.com/examples/M81M82/index.html#Noise_ReductionApplying noise reduction and sharpening at the same time is possible and sometimes useful with MMT and MLT. However, normally this only works for high-SNR data. This clearly excludes most linear deep-sky images. So better apply noise reduction only.
