Diagnose 25 of 29 Processing

Dark halos

A dark ring surrounds stars, or runs along the edges of bright structure.

At 100 %: Stars against nebulosity. The ring is darker than the background beside it.
Causes, most likely first: Over-sharpening
Sharpening raises contrast at edges. Pushed too far, it digs a dark trough beside each bright edge.
FixReduce the sharpening strength until the ring disappears, and keep the stars out of it.
Deconvolution artifacts
Deconvolution pushed past what the data supports leaves rings around stars.
FixBack off the deconvolution strength and check stars at full size after each change.
Star-removal residue
Star removal can leave a dark trace where a star was, and adding the stars back does not always cover it.
FixCheck the starless image at full size before recombining. A dark ring there survives into the final image.
Looks like, but isn't: Halos around bright stars
An optical halo is bright and is in the raw sub. A processing halo is dark and appears after a processing step.
One frame can't tell you: The finished image shows the ring. Which step made it needs your processing history, undone one step at a time.
How to confirm the fix: Stars that sit on the background with no darker ring, at full size.
Sources: No outside source is cited here. The signature, the causes and their order are the atlas’s own, from imaging experience. The order is a judgment of likelihood; no measured frequency is behind it.

In the symptom picker

Stars have a dark ring around themweight 3
It appeared after sharpening, deconvolution, noise reduction or star removalweight 2