Interesting to see even the CHART32 team isn't immune to processing artefacts and the need for sharpening.
Anyway, it's quite a complex topic Allan, as I'm sure you know. If discussed thoroughly, it would cover a significant number of processing techniques. So here are just a few of my quick thought bubbles and comments:
- Worms are just enhanced noise - so don't sharpen your noise if you can avoid it!
- Worms are more commonly seen in background and low signal areas where they are very easy to create
- If you have noisy high signal areas - acquire more data!
- Deconvolution is your friend when your stars are over sampled and larger than you want. If stars are well sampled or under sampled I'd consider it a crime to use decon
. Decon is also good for carefully and selectively sharpening nebulous detail in high signal areas.
- Decon readily produces enhanced noise artifacts (aka worms
) unless your low signal areas have very little noise to begin with ie: you have megadata or very low noise cameras etc. Note: If you overdo decon in high signal areas they can also develop worms.
- Various contrast / edge enhancing "sharpening" techniques and tools also produce noise artefacts - mostly ugly edge transistions and an "unnaturally" contrasty blobby look. Edge transitions in a good astro image should be smooth (mostly).
- Noise mitigation techniques and tools typically DON'T produce worms. It's more likely they'll produce mottling. However, severe and full frame contrast enhancement will make existing worms very visible.
In any case, and I can't stress this enough, appropriate masking becomes your best friend when dealing with noise and sharpening. Having said that, over sharpening high signal areas and then masking them off from the rest of the image can yield an obviously fake differentially sharpened look.
As to what is generally accepted, I'd say good sharpening should be gentle and not be noticeable at full res to anyone other than the author. Of course, pixel peeping will reveal even the most minor issues and is a disease that afflicts many of us! We should resist the urge to pixel peep other people's images.
I use a multitude of techniques to varying degrees:
- Decon at the initial calibration stage (CCDStack decon is quite good). It invariably produces sharpened noise because I don't create mega-data. This is especially apparent in the background and low signal areas of galaxy shots - so I often stretch the unsharpened one and then overlay the sharpened stars and high signal areas over that with masking
- Noise mitigation and background smoothing. Always with masking. PS Reduce Noise, Neat Image Reduce Noise, my own background flattening Actions for background blotchiness (Coarse, Medium and Fine)
- Contrast enhancement. I use Unsharp masking (my own PS action), Smart Sharpen, recursive high pass filtering (my own PS action), Astra Image wavelet (sparingly), APF-R (Absolute Point of Focus developed by Christoph Kaltseis - my own PS action implementation) and micro curve adjustments
Cheers, Marcus