Weird grid in image background- clearly not bias artifact

Questions and answers about processing in StarTools and how to accomplish certain tasks.

Weird grid in image background- clearly not bias artifact

Postby Soundologist » Sun Jun 09, 2019 6:15 pm

Hi all,

The linked images (here and here) are examples of recent works of mine which show an odd pattern in the background: a grid of brighter and darker lines running at a slight diagonal to the image. This grid is something that has troubled me for a few months and I want to know if StarTools can get rid of it. It appears to come from my stacked data, but isn't an artifact from my bias frames. The bias signal on my camera shows only as a bunch of horizontal lines that aren't gridded in a regular way.

I have recently started using kappa-sigma clipping in DeepSkyStacker and am also wondering if that makes sense as a potential cause of these weird grids. Either way, if StarTools has a way to process them out, I would love to know.

Thanks,
Rob
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Re: Weird grid in image background- clearly not bias artifac

Postby admin » Mon Jun 10, 2019 3:11 am

Hmmmm... That's odd indeed.
Do you dither between frames?
If you stack with "super pixels" as your debayering method, do the artifacts go away?
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Re: Weird grid in image background- clearly not bias artifac

Postby Soundologist » Mon Jun 10, 2019 4:25 am

Yes I do dither and I use mono, so I don't debayer at all. Should I still check a debayering mode?

Also edit- It does seem to not appear anymore when kappa-sigma clipping is unchecked. I can do that in my stacking, then, but preferred being able to recover data where a faint satellite flew through the corner. Do you think there would be a way to process the grid out in ST or is my best bet simply to go back to my old way of stacking?
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Re: Weird grid in image background- clearly not bias artifac

Postby admin » Tue Jun 11, 2019 2:31 am

Soundologist wrote:Yes I do dither and I use mono, so I don't debayer at all. Should I still check a debayering mode?

No. :) That can't be the problem then.

The only other - remote - possibility is the presence of interference affecting the signal (power supply or cable shielding issue).

Also edit- It does seem to not appear anymore when kappa-sigma clipping is unchecked. I can do that in my stacking, then, but preferred being able to recover data where a faint satellite flew through the corner. Do you think there would be a way to process the grid out in ST or is my best bet simply to go back to my old way of stacking?


Any outlier rejection stacking algorithm (including Kappa-sigma!) should remove satellite trails...
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Re: Weird grid in image background- clearly not bias artifac

Postby Soundologist » Tue Jun 11, 2019 2:57 am

Okay, so what I understand is that I can use another outlier rejection algorithm and see if the grid lines disappear.

Is there any mode in StarTools that can calibrate the background and even this out? Is there a wipe profile like a certain set of settings for the amp glow preset?
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Re: Weird grid in image background- clearly not bias artifac

Postby admin » Wed Jun 12, 2019 5:56 am

Soundologist wrote:Okay, so what I understand is that I can use another outlier rejection algorithm and see if the grid lines disappear.

Outlier rejection algorithms should get rid of satellite trails. I'm not sure if they would get rid of the pattern (it totally depends on what is causing the pattern!)
Is there any mode in StarTools that can calibrate the background and even this out? Is there a wipe profile like a certain set of settings for the amp glow preset?

Wipe only really deals with gradients (e.g slow moving undulations in brightness). It does not remove patterns like these unfortunately; they should really be dealt with at the acquisition stage.

Let us know how you get on; I'd be really interested to learn what is causing the pattern!
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