Strange persistent structure/artifact in old (2025) subs — what could it be?

Artifacts Help Needed
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Juan Pablo De la Cruz avatar
Juan Pablo De la Cruz avatar

Hi everyone,

Quick (and kind of funny) question.

I revisited some March 2025 data of the Dolphin Head Nebula—only 13 × 180s frames. A few days ago I resumed the project and decided to register/align those 2025 subs to my new 2026 data so I could compare (and possibly combine) them.

After alignment, I noticed a weird structure/artifact that I don’t see in my 2026 frames. I’m attaching a screenshot and a short video animation made from the 13 subs.

What’s odd:

  • It doesn’t look like dust to me.

  • It doesn’t shift in a way that matches dithering (at least not consistently).

  • It seems to persist through the short sequence.

Has anyone seen something like this before?
Any ideas are welcome.

Thanks!

SH2_308.mp4📷

Blink00013.png📷 Blink00013 - Copy.jpgBlink00013 - Copy.jpg

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Brian Puhl avatar
Brian Puhl avatar

That’s a reflection from a nearby star. It’s bouncing around because you’re dithering. Since it’s a reflection, the movements will likely be opposite of your dithers. For something like this, I’d generally look for your culprit 180 out from the reflection. I.e. if it’s in the bottom right, then your culprits probably top left.

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alpheratz06 avatar
alpheratz06 avatar

I support Brian answer, it’s something that looks out of focus at first sight , but that’s makes no sense with respect to the focus state of the rest of the field.

Looking on a single “unflattened” sub before allows to eliminate an out of focus stuff ( like dust somewhere) : this should be evenly visible on several raw stubs. If it doesn’t show up, it is something which does not stack ==> moving artefact ==> reflection is a very good candidate.

What did change between 2025 and 2026 in the optical train? Upgrading a filter quality might have seriously attenuated the issue

Juan Pablo De la Cruz avatar
Juan Pablo De la Cruz avatar

Thanks, Brian and alpheratz06! I’d never encountered this kind of reflection/ghosting before, so your explanations really help. Unfortunately these are subs from about a year ago and my optical train has changed since then, so I can’t easily reproduce or troubleshoot it now. I’ll keep the frames as a fun souvenir 😄 and if I ever see something like this again, I’ll know exactly what to look for. Thanks again for clearing it up!

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Frédéric Tapissier avatar
Frédéric Tapissier avatar

Brian Puhl · Jan 24, 2026, 06:05 PM

That’s a reflection from a nearby star. It’s bouncing around because you’re dithering. Since it’s a reflection, the movements will likely be opposite of your dithers. For something like this, I’d generally look for your culprit 180 out from the reflection. I.e. if it’s in the bottom right, then your culprits probably top left.

I agree with this conclusion

Juan Pablo De la Cruz avatar
Juan Pablo De la Cruz avatar

I reassembled the imaging train using the same components and shot the same target again and the reflection showed up once more with the O3 filter. Since it’s clearly repeatable, I’m planning to slightly rotate the filter to shift the ghost without changing the framing of the subs.

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