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Software to analyse star shape distortion, create a warp map and correct?

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#1 Tonk

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Posted 14 April 2019 - 09:40 AM

Does anyone know if there is any post processing astro imaging software that can correct oval star shapes due to lens distortion, non CCD orthogonality   etc. and warp the image so that the stars are made round?

TIA


Edited by Tonk, 14 April 2019 - 04:25 PM.

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#2 han59

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Posted 16 April 2019 - 02:35 AM

As far I'm aware there is deconvolution  and morphological filters to improve star shape but it seems hard work and your creating few or many artifacts. It will improve the image modestly. The image will be better but never as good as unproblematic image. Some have experimented with combining a deep sky survey image with there own image.

 

What your creating with these tools is going into the direction of art rather then an image.



#3 Tonk

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Posted 16 April 2019 - 06:45 AM

I think you misunderstand. This has nothing to do with "art".

 

My optical system has a significant problem with astigmatism in just one corner. Its at a remote site in another country so it is uneconomical at this stage to retrieve it for possible repair. (Expensive down time)

 

I just wondered if anyone has written an easy to use analysis / correction program that was available to apply a warp correction given that there are a number of papers in the scientific literature dealing with the very problem of lens defect correction (largely for high end photo stitching). The interesting thing is the methods need a calibration target to be imaged to characterise the lens fault before a warp map can be created. For astrophotography the calibration target will be the stars in the image - we known these should be round so we can go from here re shape distortion. Also position distortion can be correlated via plate solving star databases.

 

If no one has done this specifically for AP then I'll have a go as I worked with image processing algorithms when I worked as a developer at a university based company - just not yet sure how difficult it will be. The easiest problem to identify and fix would be CCD chip non orthogonality .. the rest is hard.


Edited by Tonk, 16 April 2019 - 07:01 AM.


#4 Tonk

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Posted 16 April 2019 - 06:54 AM

An example research poster dealing with the problem via thin spline warping. (A method also used for gradient removal in AP) http://www.sc.fsu.edu/images/stories/xpo/2013/cameron-berkley2013.pdf

 

I have a number of full papers relating to this problem.


Edited by Tonk, 16 April 2019 - 07:04 AM.


#5 han59

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Posted 16 April 2019 - 12:22 PM

With astigmatism you have to correct in X,Y and for out of focus.

 

You can display X, Y warping using plate solving. I have demonstrated this here (sorry in Dutch), look to the image:

 

https://www.astrofor...olving-aantonen

 

However where is the original center of the star when it has asymmetrical comet like shape? You could measure and replace all star flux together at the "center of gravity." with nice a 3D Gaussian shape of an appropriate diameter. But the star position will have probably a small offset in X, Y due to the original asymmetrical star shape. And to make all stars the same you probably have to do that for all stars. It will be very difficult to create smooth transitions since stars will fade away slowly from the center of gravity in the background noise. So a correction program would have to do the following:

 

  1. Detect and measure the star total flux and center position.  Measure center position is easy for a round star but probably more inaccurate for comet shaped stars.
  2. Remove all star flux from the image.(This could be most difficult part)
  3. Recreate the star shapes using the same flux amount at the original star position. (or using a star database)

 

This has also to work for faint and noisy star images.


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#6 catalogman

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Posted 16 April 2019 - 05:12 PM

Would the star rounding plugin for GIMP help?

 

http://www.hennigbua...georg/gimp.html

 

--catalogman



#7 han59

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Posted 17 April 2019 - 02:16 AM

The GIMP plugin looks interesting, installed it, but it doesn't work for my test image. It should work on tracking errors, but my test image stars are elongated only in the corners with different orientation and severeness.

 

According this 2018 discussion, lens distortions (x,y) can be fixed using astrometrical (plate) solving. However the star shape correction is much more difficult and should be adapted (PSF angle) for each corner:

 

https://pixinsight.c...p?topic=12053.0




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