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Help .. huge halo

Sky-Watcher Explorer 200PDS ZWO ASI1600MM Pro Acquisition Deep Sky Artifacts
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Andy Wray avatar
Andy Wray avatar
I decided to try and image the horsehead nebula tonight and for the first time experienced a huge halo around a star.  Can anyone give me any advice as to what may be causing it please?  FYI:  this is a skywatcher 200Pds newtonian with skywatcher coma corrector + ASI1600MM Pro. See picture below:
andrea tasselli avatar
andrea tasselli avatar
It's a reflection of the un-focused ghost of Alnitak or otherwise said an image of the inlet pupil of your scope. Typically, it's your CC's fault.
Andy Wray avatar
Andy Wray avatar
andrea tasselli:
Typically, it's your CC's fault.


Thank you for replying.  I think you are probably right about the CC being the culprit as the only other glass in the optical train was a filter and I can't imagine that creating such a large halo.  I'm not sure that there is anything I can do to minimise that effect rather than just keeping away from really bright stars ... do you have any other view?
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Joey Conenna avatar
Joey Conenna avatar
While not ideal, some techniques can help reduce this type of halo in post. Below is a link to one, and an example of my before and after.

https://digitalstars.wordpress.com/2019/10/27/tutorial-how-to-eliminate-star-halos-in-pixinsight/



andrea tasselli avatar
andrea tasselli avatar
Andy Wray:
Thank you for replying. I think you are probably right about the CC being the culprit as the only other glass in the optical train was a filter and I can't imagine that creating such a large halo. I'm not sure that there is anything I can do to minimise that effect rather than just keeping away from really bright stars ... do you have any other view?


There are ways to remove or attenuate the halos in post-processing but no, there isn't much you can do other than avoid those bright stars or change the CC with one with better anti-reflection coatings.
Concise
Mike Jaworski avatar
Mike Jaworski avatar
I had to deal with this same exact issue as well.  I was also using an asi 1600.  Turns out it was the filters, though I initially thought it was the CC.

Read more here.

I did some post processing to remove some of the artifact even after adjusting my spacing.  I wrote a little of it here.