Crescent / Fingernail shape artifact on Master light frames

Help Needed
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Ilya avatar
Ilya avatar

I recently started encountering some weird artifacts on my images while post-processing. After I finish stacking my data across individual filters L, R, G, B, I notice the master frames each have this crescent-shaped artifact that only gets worse the further I go down the processing workflow. I’ve already applied a new set of Flat frames to address my dust spots, so I don’t really suspect this could be a dust artifact. But I’m sort of at odds figuring out how to solve for this.

Does anyone have any idea what this might be? Attached below are some examples.

Blue filter Master lightimage.pngRGB combined imageimage.png📷 image.pngimage.png

Equipment used:

Camera: ZWO ASIAir 2600MM Duo

Filters: Antlia Pro LRGB

Telescope: Skywatcher Quattro 250p 10”

andrea tasselli avatar
andrea tasselli avatar

A reflection in the image train.

Ilya avatar
Ilya avatar

andrea tasselli · Apr 6, 2026, 06:05 PM

A reflection in the image train.

If it were a reflection in my image train, wouldn’t that mean I could see this artifact on individual subs as well? The problem is I can only see this artifact on the master lights, but not on the individual subs prior to stacking.

Well written
andrea tasselli avatar
andrea tasselli avatar

Not an obvious one, apparently. Hard to say without seeing the masters unprocessed. But it’s there.

Tony Gondola avatar
Tony Gondola avatar

It’s pretty faint on your master frames. I wouldn’t be surprised that you can’t see it in your subs. You see this all the time in the fainter parts of your subjects. Clear after stacking but invisible in a single sub. Anyway, yes, it’s an internal reflection. Faint but that’s what it is.

Well written
Brian Puhl avatar
Brian Puhl avatar

Stack your frames without calibration.

Compare your flat masters for movement.

While I’m not ruling out reflection, this looks more to me like a shifted dust mote. Either it’s physically shifted during your session and new flats aren’t lining up still, OR your filter wheel is not aligning exactly.

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