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Semicircular Blue halos

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Hi all,

I noticed these semicircular blue halos on the bright stars. This image was taken on an Evoguide 50ED. They are all on the same side.

Is it chromatic aberration?

Is there something that can be done to reduce it either in set-up or processing?

Or am I expecting too much from such a cheap OTA?

Many thanks



Edited by vernmid
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It looks more like atmospheric dispersion as you have separated red and blue at different sides of your stars. They are also separated vertically as blue in this case would be higher in the sky than red if you shot this while the horsehead was close-ish to the meridian. It happens because the target is too low in the sky (below 40 degrees is bad, above 60 is where it starts to not matter), which is not something you can do anything about but its fixable in post.

You need to extract RGB from the stack and register the frames against each other and it may fix the issue by properly aligning the colour channels. You will still be left with the blue halo because of the chromatic aberration in the fast doublet lens, but it should be more symmetric at least.

I will assume you used a colour camera here as that is the how you capture all the colours at once. You have another option which fixes this issue more robustly and from the source and leaves no artifacts behind, but its a lot more work to do. You first calibrate all the frames WITHOUT debayering them yet, then you can split your frames using the CFA_split or seqsplit_CFA commands in Siril (PixInsight can too if you have it). This splits each of your raw OSC subs into 4 mono ones : 1 red, 2 different green ones and 1 blue. Then you can choose one of these frames as a reference frame for registration and stack all the frames from the different channels to that. That obviously means all the RGB separation issues are removed as every frame is distorted and transformed to correctly fit the reference frame, leaving you with mono stacks that colour align perfectly!

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Thank you @ONIKKINEN

That's excellent advice.

I shot the image last night and I think it was at 36deg maximum and yes the session spanned the horse head crossing the meridian. Camera was an ASI585MC, so yes an OSC.

I hadn't really spotted the opposite red half halos until you pointed it out but all makes sense.

I will try both your suggestions and let you know how I get on.

Thanks again for taking the time to help me 👍

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