Ring Artifact after Background Extraction in Pixinsight - Forums

Topic starter

Oct 8, 2020

Dear community

I recently got my new EQ6-R mount that finally allows me to take some nice image with my 10" Newton. So far everything works great, but I noticed a specific issue with my current setup. As you can see in the picture below, I get a kind of ring or donut at the edges of the image after doing the background extraction.

I just spent several hours trying both the ABE & DBE process, playing around with every single parameter in both modules as well as with different sample placement attempts in the DBE process. I simply can't get rid of the problem. Please note that I just quickly generated the below picture for this post, so not all DBE samples have been well placed here. But it illustrates well what I'm talking about.

As you can see in the second picture, the artifact doesn't seem to be present before the background extraction. This is strange. Another thing I noted is that I don't see that problem if I use the same camera with my 135 mm Samyang lens. For all pictures I took with that wide-field setup, I get perfectly flat images after the DBE process, also in the extreme corners. Only the combination of the same camera (Nikon D3300) + ES HR coma corrector + 10" Newton suffers from that kind of artifact. Could it com from an internal reflection in the coma corrector? As long as I can't solve it, I have to crop my images quite a bit in order to use only the parts that are not affected. This is ok for small objects like M27, but hurts for bigger ones. Until now I took three images in three different nights with my new setup. All of them showed the same artifact.

Do you have any idea what might cause it? I really would like to be able to use my full FOV again!

Engaging

Steve Ludwig

Topic starter

Oct 8, 2020

Small addition. Today I took some flats and just did a background extraction from the master flat. Same issue. I think the bright parts originate in some internal reflection. This is then in turn confusing the Pixinsight background extraction algorithms so that it doesn't work in the extreme corners, which remain relatively dark. I will try to do some further flats without the coma corrector tomorrow.

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Michael Timm

Oct 8, 2020

Hey Steve, this could be a reflection, especially your flat suggests so. Can you give me access to
1) a stacked image of your lights without applying flats
2) a stacked image of your lights with flats applied
3) a stack of your flats (also known as Master Flat) :-)
Be aware that if you change something on your optical train (like removing the coma corrector) your flats will be unusable for the image taken with the coma corrector.

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Steve Ludwig

Topic starter

Oct 9, 2020

Hi Michael,

unfortunately I don't have flats that match with a particular image as I normally don't use correction frames at all. The flats in my last post, I only did to investigate the potential root cause. Meanwhile, I took another series of flats without the coma corrector attached to my scope, so only camera and scope. The image below shows the resulting master-flat after DBE. As you can easily see, the problem seems to have disappeared, which is a strong indication that the images above might really suffer from an internal reflection related to the coma corrector.

Does anyone else notice such issues with the "Explore Scientific 2" HR Coma Corrector"? One more thing that I could try is to adjust the backfocus. With the included adapters you only reach the standard backfocus of 55 mm, which ES calculated as optimum for a scope with 800 mm focal length. According to the documentation that came with the corrector, the optimum backfocus for my focal length of 1270mm would be close to 56.5 mm. Might it be possible that the lacking 1.5 mm are responsible for the artifact? If yes, the next difficult task would be to find suitable spacer rings.

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Michael Timm

Oct 9, 2020

Without any doubt, the coma corrector is the source. Good luck and CS to you!

Steve Ludwig

Topic starter

Oct 9, 2020 · Edited: Oct 9, 2020

I found a solution: http://trappedphotons.com/blog/?p=756#WithoutSubframes

This article describes how to generate a synthetic flat using Pixinsight. Instead of using individual lights, I simply applied "Multiscale Median Transform" to the final stacked image, extracted the big structures, made the resulting image containing those structures smooth with the clone stamp tool (removing M27 and remaining bright stars) and used "pixel math" to subtract the resulting synthetic flat from the original stacked image. After increasing saturation a bit a got the picture below. That's amazing! So easy! I will probably never use the DBE process again as the synthetic flat works so much better. All artifacts are gone and the image is completely flat!

Edit: This is the artificial flat I generated:

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Yannis Doukakis

Oct 11, 2020

As people said before, double-check the Coma Corrector distance. I had worse artefacts, having even the obstruction visible, only to find out that by wrong addition - and I am mathematician

- I put the Baader at 49mm instead of 57mm. Adding a 2" tube solved the problem.
Link to flat

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Erlend Langsrud

Oct 11, 2020

I guess there's something wrong with your flatfield-calibration. Maybe the focuser was in another position when you took the flats. Or something.

Yannis Doukakis

Oct 12, 2020

Please also check the following, which may disturb the flats.
Many Newtonians do not like to have their flats taken, with a paper/box/t-shirt/whatever attached directly to the end of the tube. Sometimes, light gets reflected from the tube/secondary/secondary-holder directly into the focuser, bypassing the normal path primary->secondary->focuser in weird angles and destroying the flat generation. In this case you need some distance between the flat panel and the tube end.
Try the following to see if the situation is improved. Point your scope to any flat wall that may be around, even if the wall is fairly dark. Adjust exposure to take a few flats and use these flats for adjusting your images. If the wall is not evenly lit, you may end-up with just a gradient, not the nasty circles you are fighting with.
Alternatively if you have, put an extra dew shield in front of the telescope and put the flat generator (paper/t-shirt/lightbox) on top of it. Point the telescope to the zenith to keep balance.
Or you can use the sky.

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Peter_S

Apr 17, 2023

Yannis Doukakis:
As people said before, double-check the Coma Corrector distance. I had worse artefacts, having even the obstruction visible, only to find out that by wrong addition - and I am mathematician - I put the Baader at 49mm instead of 57mm. Adding a 2" tube solved the problem.
Link to flat

Yannis - thanks a lot! I had the same issues and that is the solution1
cs Peter

Erlend Langsrud

Apr 18, 2023

I think one common reason is straylight. Straylight is added to the image. Flats are applied as division because it is meant to handle difference in sensitivity.

Lets say the signal is 1 and the straylight is 0.1 for a given pixel. The value of the flat is then 1.1 for the same pixel.

Flats will "sort of" take care of straylight because 1.1 / 1.1 is equal to 1.1 - 0.1. It seams like division achieves the same as subtraction, and it does.

Almost.

The problem is that the spectrum of your light panel is not the same as the night sky. This is no problem for true vignetting, because light falloff is proportional for all wavelengths. The problem with straylight is that the different wavelengths have different reflectivity.

TL/DR:

A bright center might look like vignetting, but if is is not handled by flats and has sharp edges it is probably straylight IMHO. Unlike true vignetting, it is hard to remove by background extraction because of the relatively sharp edges. A dark circle will typically remain.

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