Strange artifacts

Questions and answers about processing in StarTools and how to accomplish certain tasks.
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JDShtoots
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Joined: Tue Sep 29, 2020 12:14 am

Strange artifacts

Post by JDShtoots »

Hello, I have my first question about an issue I could not resolve on my own. I have processed just over a dozen or so images since starting this hobby last year and this was a first for me.

I'll cut to the chase, here is the issue, I have a weird color "vignetting" in my stack that I have never seen before and was difficult to remove. This is a screen shot of the completed Wipe just before hittin Keep
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Here is the Cocoon from a month earlier, same point wiped ready to hit keep. I show you the Cocoon, since this has been typical for ALL my imaging prior the the Iris. I've always breezed through wipe without issues. I did in the Cocoon too, but Iris was a killer. Both of these were shot in the same manner, same equipment, substantially similar location in the sky, I really didn't expect any issues. I started the Iris session at an Altitude of 62° and ended the night 6 hours later at 37°. I am at a longitude of 40°. Booth sessions had new Dark, flat and Bias frames, with the only difference that The Iris was shot at ISO 100 instead of ISO 200. Thinking I had a calibration issue, I went through and replaced all my cal frames on set at a time, but it was in the subs. I even stacked a half hour of subs w/o cal frames at the start and middle of the session just to see, and it's in there. Could this be the California forest fire smoke?? I tried for a night when it was forecast at a min. Or is there something different about this object that I need to process uniquely. I tried masking off just the center and it really didn't seem to help me much.
Image
In the end I am fairly happy with my result, but I feel I sacrificed some signal when minimizing the LP in Wipe?
Image
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admin
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Re: Strange artifacts

Post by admin »

To properly diagnose what might be going on, would you be able to upload the stack somewhere?

The diagnostics "courtesy" AutoDev in Wipe intentionally exaggerates unevenness and discrepancies on purpose to visualise any sort of defect/discrepancy it can find. So, in the end, such defects may indeed not be a massive issue (if at all) in the final result (which is typically stretched much less).

Typically color discrepancies like these between the channels can sometimes be caused by dark anomalies (bumping up the Dark Anomaly filter may help in that case). However seeing them in the form of faint concentric rings is rare. They may also be caused by quit ehavy light pollution in combination with vignetting, in which case gradients may change too quickly for Wipe to model precisely enough. Increasing the Precision will fix this.

Do let me know if this helps!
Ivo Jager
StarTools creator and astronomy enthusiast
JDShtoots
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Joined: Tue Sep 29, 2020 12:14 am

Re: Strange artifacts

Post by JDShtoots »

Thank you Ivo, I will put the stack up on Dropbox.
I understand the the initial Autodev is excessive, but then wipe had problems. And I never saw this affect before. And I have never had to use the Vignetting feature on my work since the flats do a good job, usually just a gradient wipe. Here you can see the corner drop off and like I originally said, I tried a new set of flats for that reason too. It is a weird issue, and if it were my first time using this setup I would think nothing of it but I have nearly a dozen with this. That's why I was wondering about the smoke in the upper atmosphere, that is a new variable.
In wipe, in the end I tried a LOT of variations, but landed on a Color and Brightness mode, 93% aggressiveness, 0% dropoff, 256*256, 1pixel Dark Anomaly and 101% Corner Aggr. I did move the dark anomaly setting around, well with all of the others too, this seemed to work best.

I live in a Bortle 5 area, and I didn't notice any new streetlights or anything. I also started this session looking very high, just over an altitude of 62° and in a direction away from most of the LP sources around me. I am not ruling that out though, I may have just missed a new source. I also mentioned the Cocoon again, since it was taken with the exact same setup pointing in nearly the same direction about a month earlier, and it's "clean", well as much as a Bortle 5 sky can be:) This is the 10th target I have taken with this setup and the first with this issue. It is an old manual focus 500mm f4 Nikon lens on a D7200. The lens has a 6" long hood on it to help prevent LP and Dew, and I use a Dew heater.
Hopefully the dropbox links work. Note these are straight from DSS and the screen shots above have a fairly hard crop on them to remove the stacking errors on the edges, I probably removed 300 pixels all around.
Here is the Iris stack:
https://www.dropbox.com/s/26o1boj26crlc ... n.TIF?dl=0
And here is the Cocoon for reference:
https://www.dropbox.com/s/xc4bzvo6y8doe ... l.TIF?dl=0
Thank you very much.
JD
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Re: Strange artifacts

Post by admin »

Thanks JD,

I had a look and my best guess is that your flats do not match your lights - likely something caused a shift (in location or brightness) between the time you acquired the lights and the time you took your flats.

The rings are perfectly concentric and definitely of artificial origin.

I would see if the problem persists next time you go out and acquire lights (and redo your flats).
Ivo Jager
StarTools creator and astronomy enthusiast
JDShtoots
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Joined: Tue Sep 29, 2020 12:14 am

Re: Strange artifacts

Post by JDShtoots »

Thank you for taking a look Ivo! It is a puzzling dataset for me. Here's hoping the next session is clean. And yes, I will redo the flats as I always do new cals each session.

Love the software keep up the great work,
JD
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Re: Strange artifacts

Post by admin »

Thanks JD! :)
Let us know how you go.
Clear skies!
Ivo Jager
StarTools creator and astronomy enthusiast
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