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Comet stacking in Deep Sky Stacker. How does it do it?

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This has got me intrigued. I can see how standard stacking works in which the comet is fuzzy and the stars trail.

I can see how comet stacking works where the stars trail and the comet doesn't.

But I can't work out how the software achieves comet and star tracking - the "star freeze effect".

Anyone know?

Edited by Ouroboros
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The general steps are:

* Align the image twice, one time aligned on the stars and one time aligned on the comet.

* Stack both images using methods that remove pixels that remove random pixls and moving objects (median, sigma clip etc.) .

* Of the two images you get from this one will remove the stars (fixed comet, moving stars)  and one will suppress the comet (fixed stars, moving comet).

* Align an merge the images.

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Yes, probably. I have managed to do use the star freeze effect as DSS call it. It didn't produce as nice a result as just stacking normally.

I'm just intrigued as to how the software does it. I have enough mathematical understanding to see how straight forward registering and stacking works, both on the stars or comet. I can't imagine the processing that enables these two images to be combined. A hand waving explanation is probably sufficient.

I once did a combination in Photoshop of a manually stacked comet picture against a manually stacked star field. It was possible to stitch the two images together because the region around the comet was conveniently free of stars. But I assume that's not how DSS do it.

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