What are these artefacts?

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Nyx
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Joined: Mon Jul 27, 2020 8:11 am

What are these artefacts?

#1

Post by Nyx »

A couple of days ago I captured M101 via Live Stacking (1h). I'm overall pleased with the result, though I cannot explain the highlighted artefacts in the final stack:

https://drive.google.com/file/d/1FsKYF5 ... sp=sharing (13MB Tiff)

My best guess is that they are hot pixels spread around due to dithering. But, I used sigma clipping and darks, both of which should take care of hot pixels.

Any clues?

Edit: I just noticed that the patterns of the dots is identical. I think the theory of hot pixels being moved around via dithering is probably true. Still, it doesn't make sense considering the use of darks (temperature/gain/exposure matched master dark applied in preprocessing) and sigma clipping.
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Re: What are these artefacts?

#2

Post by admin »

Hi,

I agree with your analysis that these are to do with hot pixels being spread around via dithering. It's probably worth having a look at the light frames and dark frames in detail at roughly those pixel co-ordinates to see if there are some hot pixels in the light frame that are not in the dark somehow (as that seems the most likely cause). Sigma clipping may or may not help - it depends on the settings for the sigma clipping and just how 'hot' those pixels are. When you add enough stretch you can end up pulling very tiny changes in pixel brightness into view, so it's hard to be sure.

If you saved the individual frames then you could re-stack using the folder monitor camera with different sigma clip settings to see if that helps. Also, there is a new feature coming in today's SharpCap update that is a hot pixel removal option that is an alternative to using dark frames - that might help you in a reprocessing run. More on that in a separate post later...

cheers,

Robin
Nyx
Posts: 13
Joined: Mon Jul 27, 2020 8:11 am

Re: What are these artefacts?

#3

Post by Nyx »

admin wrote: Mon Jun 06, 2022 12:57 pm Hi,

I agree with your analysis that these are to do with hot pixels being spread around via dithering. It's probably worth having a look at the light frames and dark frames in detail at roughly those pixel co-ordinates to see if there are some hot pixels in the light frame that are not in the dark somehow (as that seems the most likely cause). Sigma clipping may or may not help - it depends on the settings for the sigma clipping and just how 'hot' those pixels are. When you add enough stretch you can end up pulling very tiny changes in pixel brightness into view, so it's hard to be sure.

If you saved the individual frames then you could re-stack using the folder monitor camera with different sigma clip settings to see if that helps. Also, there is a new feature coming in today's SharpCap update that is a hot pixel removal option that is an alternative to using dark frames - that might help you in a reprocessing run. More on that in a separate post later...

cheers,

Robin
All values related to sigma rejection are left untouched (can't recall what the defaults are). I think, I've only maxed the number of alignment stars, but I doubt that's of any relevance here.

It's been like 2-3 months since I've created my dark library. I was under the impression that my darks are still "fresh", but I can try redoing them in case new hot pixels emerged.

Regarding dithering, sigma clipping and satellite trails (I have like 4-5 of the in my M101 image), are there any best practices? If the initial frame count is set to 5 (I think that's the default), should I keep resetting the stack until the first 5 frames are free of satellites? What about dithering and the initial frame count?

I cannot say with certainty what happened there, but I know I set dithering every 12 frames (10s subs), so dithering didn't occur during the first 5 frames.

From what I understand, if the hot pixels aren't removed from the master dark, there is no way to get rid of them using dithering/sigma clipping while live stacking. That being said, I thought sigma clipping should eliminate satellite trails, right? With 360 dithered frames there shouldn't be any satellite trails left, or?
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