Author Topic: How to remove ringing around my brighter stars  (Read 2763 times)

Offline rtemen

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How to remove ringing around my brighter stars
« on: 2017 November 04 06:45:01 »
I followed the tutorials on Light Vortex by Kayron Mercieca where he processes Andromeda.
When I finished, there is some significant ringing, especially around some of the start in front of the galaxy.
Could someone guide me as to where in the process I should have done some steps that would have removed the ringing?
I saved my project, so I should be able to backup to the proper step and work on the ringing.

Thanks,

Rich

Offline pfile

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Re: How to remove ringing around my brighter stars
« Reply #1 on: 2017 November 04 09:07:03 »
i don't know the steps in the tutorial, but you usually get ringing during deconvolution or sharpening.

rob

Offline rtemen

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Re: How to remove ringing around my brighter stars
« Reply #2 on: 2017 November 04 10:18:54 »
Hi, Rob.
Thanks for the reply.
I am fairly new to PixInsight, so I do not know what a lot of the tools are, or what their names even mean.
So at this point, are you able to tell me anything about deconvolution as to about where in the process it should be used?
Like, does it need to be on a linear or non-linear picture?
Any hints on what general steps it should be used between?

If you know of a good tutorial on deconvolution that you could point me to, that might help as well.

Thanks,
Rich.

Offline pfile

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Re: How to remove ringing around my brighter stars
« Reply #3 on: 2017 November 04 11:32:17 »
deconvolution should be performed on a linear image. it's kind of an advanced process; it is an attempt to recover the "true" image which is unperturbed by various optical effects (atmosphere, telescope). it's prone to ringing artifacts; there's a whole art to configuring the deringing parameters to keep it in check.

i'm sure kayron has an article on it. if not, maybe pixinsight resources or david ault or jon rista has something on their website.

rob

Offline wimvb

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Re: How to remove ringing around my brighter stars
« Reply #4 on: 2017 November 04 16:36:58 »
Maybe this tutorial can help:

https://pixinsight.com/forum/index.php?topic=4321.msg30342#msg30342

and this:

http://www.pixinsight.com/examples/M81M82/index.html

I used to struggle with deconvolution, and overdid things because I didn't really understand what I was doing. I find Juan's explanations still the most straight forward.

Hope this helps,
Wim

AZ EQ6 GT + EQ3 PRO, SW 150P-DS, SW MN190, ZWO ASI174MM-Cool, Pentax K20D (retired),
SW ST80 + ZWO ASI120MM + Lin_guider

Offline rtemen

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Re: How to remove ringing around my brighter stars
« Reply #5 on: 2017 November 07 07:10:05 »
While proceeding with the tutorial, I am at this point, where he indicates the first pass at noise reduction.
Here is a shot of my image at this point.
https://www.dropbox.com/s/a471p9p7la9iy6r/BlueRinging.jpg?dl=0
There is some rather obvious blue ringing around many of the stars.
This tool does have a De-Ringing section.  I have tried to enable it and tweak the numbers a bit, but I obviously do not know what to do.
Is it already too late to try to use this Deringing tool in the MultiscaleLinearTransform process?

Thanks for any guidance.

Rich

Offline rtemen

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Re: How to remove ringing around my brighter stars - latest update
« Reply #6 on: 2017 November 08 09:30:09 »
I started going through the tutorial again from the beginning.  I was trying to watch the results of each step.
Here is what I have found.
In Kayron's tutorial, in Section 5 Boosting colour and fine detail contrast in the target, I have noticed the big difference.
In the section:

Let us now boost the colour in the galaxies just like we neutralised the colour in the background earlier, using the MultiscaleLinearTransform process. Click the Reset button, select 8 under Layers and select Chrominance (restore CIE Y) under Target. We again address the large wavelet Layers 5 to 8. This time however, the Bias values we enter are positive and nowhere near as aggressive. I will enter 0.150 for Layers 5 and 6 and 0.200 for Layers 7 and 8?.

When I apply the MultiscaleLinearTransform, the blue rings around the start really jump out!

Here is a screenshot.
https://www.dropbox.com/s/vdbjb2qmc7b8ms4/BlueRinging2.jpg?dl=0

Any suggestions will be appreciated.

Rich


Offline pfile

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Re: How to remove ringing around my brighter stars
« Reply #7 on: 2017 November 08 09:37:15 »
OK the MMT process can also do this. i am guessing here, but i think what you have going on is a DSLR and a telescope that can't bring all the colors of light to focus at the same plane. this is common for certain types of refractor. if you look you'll see that your blue stars are fatter than the green and red even from the linear stage. this operation is essentially just highlighting the color of the blue stars where they poke out around the other two channels.

there's some chance this was caused by the lack of UV blocking in your imaging train - blue star bloat can be caused by UV sometimes.

rob

Offline wimvb

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Re: How to remove ringing around my brighter stars
« Reply #8 on: 2017 November 08 13:00:21 »
What you have are not ordinary ringing artefacts caused by deconvolution, but as Rob noted, a difference in focus for blue as opposed to red and green. This is common for (non-APO) refractors.
The easiest solution to your problem is to create a contour star mask that reveals the offending areas. Apply and use the colour saturation tool to desaturate the bright star haloes. Do this at the very end of your process.
If the ordinary starmask process doesn't produce the contours you need, you can try taking the difference of two star masks. In mask A you use one more layer and a higher value for the "large scale" parameter than in mask B. Then subtract mask B from mask A. Adjust the brightness and softness of this difference_mask to taste.

Hope this helps.
Wim

AZ EQ6 GT + EQ3 PRO, SW 150P-DS, SW MN190, ZWO ASI174MM-Cool, Pentax K20D (retired),
SW ST80 + ZWO ASI120MM + Lin_guider

Offline rtemen

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Re: How to remove ringing around my brighter stars
« Reply #9 on: 2017 November 09 12:06:11 »
Hi, Wim and Rob.
Thanks for this very educational information about the blue bloating and difference of focus between the color wavelengths.

Wim, I have to admit that I am very much a novice still on PixInsight.
While your description makes sense as far as using a starmask to attenuate the undesired colors, I don't have a clue on how to implement it.
My expertise level at this point is to follow a tutorial step by step and see how it turns out.
If any particular step is not optimal, I don't know enough to know how to tweak it for better results.
That being said, I do not know the steps to take to accomplish what you are suggesting.

In Kayron's tutorial, he steps through using a StarMask. Would it be possible to handle the blue desaturation at this point?
Do you know of a tutorial on stepping me through your suggested course of action?

If I do your suggested StarMask method at the end, can I use the starmask that I saved during the Kayron tutorial?

Thanks from the novice...

Rich

Offline wimvb

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Re: How to remove ringing around my brighter stars
« Reply #10 on: 2017 November 10 03:02:28 »
I will try to write down a workflow this weekend. The success of this method depends very much on the star mask, so you'll need to spend some time on making a good one.
Wim

AZ EQ6 GT + EQ3 PRO, SW 150P-DS, SW MN190, ZWO ASI174MM-Cool, Pentax K20D (retired),
SW ST80 + ZWO ASI120MM + Lin_guider

Offline rtemen

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Re: How to remove ringing around my brighter stars
« Reply #11 on: 2017 November 10 06:45:13 »
Thanks, that would be great.
I'll keep my eye open for your info.

Rich

Offline wimvb

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Re: How to remove ringing around my brighter stars
« Reply #12 on: 2017 November 10 15:16:52 »
Here's a possible solution.
(Btw, I isolated the star image from the screenshot in this thread as an example.)

Create a starmask that will include the blue halos. To include large stars, you need to increase Scale. Generally, a larger value for scale needs a lower value for Noise threshold. I used Scale= 7, Noise threshold = 0.35. Large-scale = 2, Small-scale = 1, Compensation = 2 (first attachment)

Then I created another star mask with Scale = 6, Noise threshold = 0.35 and Large-scale = 0, Small-scale = 8 (not critical), Compensation = 2.
I used Histogram transformation to strengthen this second star mask, and then Pixelmath to subtract star mask 2 from star mask 1. Done correctly, this will create a contour mask. It may be necessary to strengthen the second star mask (with smaller stars), in order to get dark cores. (second attachment)

I then applied the contour mask (ring_mask) to the colour image and used the Color-saturation tool to decrease the saturation of the blue halos. (next post)
Wim

AZ EQ6 GT + EQ3 PRO, SW 150P-DS, SW MN190, ZWO ASI174MM-Cool, Pentax K20D (retired),
SW ST80 + ZWO ASI120MM + Lin_guider

Offline wimvb

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Re: How to remove ringing around my brighter stars
« Reply #13 on: 2017 November 10 15:25:26 »
...
here's a screenshot of the color saturation tool with the image and masks (first attachment), and a before and after image (second attachment).

Caveat: be careful when using this method for halos that are over bright targets, since it will in essence create colourless halos instead.
Also, the residual halo will be further reduced when darkening the background. It may be desirable to leave a little of the colour around the stars.

Btw, also have a look at this processing example by Alejandro Tombolini:

http://pixinsight.com.ar/en/info/processing-examples/31/m101-blue-halos-reduction.html
Wim

AZ EQ6 GT + EQ3 PRO, SW 150P-DS, SW MN190, ZWO ASI174MM-Cool, Pentax K20D (retired),
SW ST80 + ZWO ASI120MM + Lin_guider

Offline msmythers

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Re: How to remove ringing around my brighter stars
« Reply #14 on: 2017 November 10 15:53:47 »
Thought I'd jump in and mention a technique I use sometimes with blue halos around stars. I use the Colormask. Here is a link to a posting where I show what I do. It's fairly straight forward.
https://pixinsight.com/forum/index.php?topic=9250.msg59525#msg59525


Mike