WBPP Integration creating star artefacts (non symmetric) - or am I wrong? - Forums

Topic starter

Feb 5, 2023

I need some help with this.
A "standard" WBPP processing of the M42 has created (to me) strange artefacts on the biggest stars in the Master Light file:

Facts: Askar FRA400 och ASI2600MC, 80 x 60 sec exp. 30 Flats and 30 Dark Flats.
To me it looks as if it is the ImageIntegration process that creates these artefacts, I do NOT see it in any of the previous file versions (fits, calibrated files, cosmetized files, debayered files, registered files). I have tried to use all the different Rejection Algorithms in the ImageIntegration but they all leave results like this.
It is only the strongest stars that are affected.
The effects get stronger when I do nonLinear stretches.
It happens with other pictures as well.
Question 1: Is the effect caused by my optical train or is it some effect of the WBPP process?
Question 2: Can I do the calibration and stacking in a different way to avoid these artefacts?
Question 3: If it is not possible to avoid the artefacts - how do I deal with them to make them less significant?
/Peter

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Roger Nichol

Feb 5, 2023

I doubt that it is a stacking artefact. I would first blink through your debayered and aligned images to see if any have any diffraction spikes from shooting through a tree branch or similar. Normally rejection would largely get rid of those though during integration, unless the reference image has them.

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Peter Christensen

Topic starter

Feb 5, 2023

I have been blinking all debayered and registered files and I do not see any reason to the artefacts - no diffraction spikes or anything alike.
You are probably right about the stacking beeing the reason. But then I think I ought to be able to see the effect in the calibrated files.
I do not understand this. Why is it only in the strongest stars???
But if I cannot find the reason - how do I remove the artefacts in all the strong stars?

jewzaam

Feb 5, 2023 · Edited: Feb 5, 2023

Very likely something in the optical train that is causing a small spike. Others may have suggestions on what, but things I can think of are small artifacts on the edge of the lens and pinched optics. I'm sure I'm wrong but :shrug:

Why you don't see it in each sub is it's faint. I had one target I took a few dozen of hours of data with and only after found there was a small smudge on my UV/IR cut filter. It didn't show up in the individual subs but when stacked there was a strong streak on one side and a weaker streak on the other (from meridian flip).

I solved it by taking a starless image into another tool and removing it by hand. I used Affinity Photo. If you don't have a lot of stars that look like this you probably can try that. I would try select-by-color, feather and/or grow/shrink the selection until it's isolated to the big stars and hopefully rounded. Then use something to focus on the bits that are outside of that round area: levels, curves, or infill.

Maybe you can change pre-processing to be more aggressive, but I don't expect that will work well given this will be present in all the subs. If you can get it to move around in the subs it might reject better. Then you could do a separate integration that's more aggressive to just get the stars (I assume it would reject any non-stars in that case so you don't want it for that). If you captured data only on one side of meridian try capturing on the other. You don't mention the sensor but if it happens to be square try rotating 90 degrees. This will put the spikes on different sides of the star, helping with rejection?

Good luck!

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Peter Christensen

Topic starter

Feb 5, 2023

Thank You jewzaam, I appreciate your answer very much. I do think you are right about the reason for the star artifacts.
You have given me a lot to think about, and I will try your suggestions for dealing with the subs and possibly more aggressive rejection adjustments.
I won't be able to rotate without consequenses as the the sensor is rectangular.

Going to the laborotory tomorrow - right now I am having an encounter with Pacman (for the first time). The encounter is very distracted by clouds.

Thanks a lot - I will come back, eventually.

/Peter

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Roger Nichol

Feb 5, 2023

One thing you could try is BlurXterminator - it may clean them up for you. If you don't have it then I could have a go for you on the integrated linear image and see what happens.

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Bob McInteer

Feb 5, 2023

Boy, this looks a lot like a problem Joey Troy had with an Askar. You can read about it in his blog here: https://joeytroy.com/fixing-pinched-optics-on-the-wo-zenithstar-61ii/
He's very helpful if you get in touch.

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Peter Christensen

Topic starter

Feb 6, 2023 · Edited: Feb 6, 2023

bobthemac: Thanks a lot - I will check it out immediately!

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Peter Christensen

Topic starter

Feb 6, 2023

Roger Nichol:
One thing you could try is BlurXterminator - it may clean them up for you. If you don't have it then I could have a go for you on the integrated linear image and see what happens.

Roger Nichol: Thanks for your reply. My first priority is to try to remedy the cause of the artefacts, and it looks as if something can be done about it.
I will definitely try your solution on the existing pictures, and might come back to you for further help regarding.
/Peter

Peter Christensen

Topic starter

Feb 6, 2023

Investigating further - and analysing.
Tomorrow night (hopefully, if the sky is clear) I will continue investigation and testing my latest theory: It might be Pinched Optics, meaning that the optical tube is causing a stress in one or more of the lenses, giving this very specific distortion of stars of a certain size.
I have been cleaning lenses to exclude this as a cause.
Will be testing the preparations tomorrow, in combination with a possibly slight relaxation of the lens mounting and the tube rings holding the scope in place.
/Peter

Peter Christensen

Topic starter

Mar 29, 2023

It turned out that my initial presumption was wrong - it was NOT WBPP creating artefacts, but something quite different. I had many creative ideas coming up answering my initial questionn, but it was the reference from bobthemac about a similar problem from Joey Troy which gave me the clue to the solution.

Pinched optics turned out to be the culprit. See Joey Troys very informative blogpost about this: https://joeytroy.com/fixing-pinched-optics-on-the-wo-zenithstar-61ii/

I followed the ideas and made a few minor udjustments to my Askar FRA400, simply loosening the screws holding the mounting rings. It turned out that I only had the problems with star artefacts when I was working at low temperatures (minus 18 deg and below), a fact that I did not consider in the beginning when I observed the problem the first times. In the cold the tube material and the mounting rings got so tight against each other (the expansion of different materials with different coefficients) from the cold that the tube was deformed, pinching the optics inside.

Loosening the screws holding the lense arrangement en the anmounting rings solved the problem.

Thanks for all the help and all the good ideas coming up - I appreciate it very much! But most thanks to bobthemac who turned me in the right direction!!!

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