How to bloat stars on purpose?

John_Gill

Well-known member
May 6, 2016
419
3
62
Durban, South Africa
Hi,

Perhaps make a StarMask that only includes big stars. You can then use CloneStamp on any other bright star/objects. Apply the mask and Invert and then using HistogramTransformation or CurvesTransformation you can make these stars appear bigger. I am sure there are other ways to do this as well...

spaceisnotblack
John
 

fredvanner

Well-known member
Apr 17, 2019
86
4
Wells, Somerset, UK
I have some ideas, and I'd be happy to experiment if you can post a solvable image (I'd just need approx. focal length, pixel size and some identifiable object or RA/Dec inside the image, so ImageSolver can work).
 

fredvanner

Well-known member
Apr 17, 2019
86
4
Wells, Somerset, UK
I've started by solving your screenshot. My plan is based on the idea of using the CatalogStarGenerator (CSG) script to generate a synthetic template of just the brightest stars, synthesised with an exagerated psf width. This can then be used as a luminance template for your "bloated" stars. This can then be coloured using the chroma channels from the peaks of the target stars (this would be simplest with a script, but I'd planned to prototype it with some iterative PixelMath). The advantage of this approach is that the "bloated" stars will have a shaped psf, so will respond reasonably to histogram stretch operations. However, I've hit a problem: CSG limits the synthesised FWHM to 100arcsec; your image is about 128 as/pix, so your desired bloated stars will be bigger than this. While a higher resolution image would help the processing, it won't solve this limit. CSG is not one of the installed script sources, so I can't easily modify this limit. I'll keep thinking about it...
 
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wuxiekeji

New member
Jun 29, 2020
4
3
I've started by solving your screenshot. My plan is based on the idea of using the CatalogStarGenerator (CSG) script to generate a synthetic template of just the brightest stars, synthesised with an exagerated psf width. This can then be used as a luminance template for your "bloated" stars. This can then be coloured using the chroma channels from the peaks of the target stars (this would be simplest with a script, but I'd planned to prototype it with some iterative PixelMath). The advantage of this approach is that the "bloated" stars will have a shaped psf, so will respond reasonably to histogram stretch operations. However, I've hit a problem: CSG limits the synthesised FWHM to 100arcsec; your image is about 128 as/pix, so your desired bloated stars will be bigger than this. While a higher resolution image would help the processing, it won't solve this limit. CSG is not one of the installed script sources, so I can't easily modify this limit. I'll keep thinking about it...
Hi Fred,
Thanks! This is an interesting approach. The screenshot is roughly centered or Orion, and Barnard's Loop, Orion Nebula, Horsehead Nebula are all in the frame. If it helps your solver or anything I've posted a higher resolution version at: https://drive.google.com/drive/folders/12DeDwD2Y3O9rp1UEVYiFphboQNU0Knpb?usp=sharing
Focal length is 21mm, sensor size is 35mm full frame, cropped very slightly after stacking (probably around 5%)

Of course I'm mostly looking for ideas of a good approach to this rather than someone to do it for me, but feel free to play with it if you are interested in doing that. :)

(I have a separate long exposure foreground image that was taken with the star tracker paused that will cover up the big smear on the left half, in case anyone was wondering.)
 

astrograz

Member
Apr 19, 2020
8
1
I would go the other way round and try to reduce all stars except the brightest ones. One possible approach would be to make a starless version with starnet, then extract the luminance from the original file, (edit: forgot to mention that you need to invert it), use clone stamp to "black out" the brightest stars, reduce the brightness and apply it as mask to the original file. Then use pixelmath to apply the starless version multiple times to the original file with the mask on it until you're satisfied with the result.

CS, David
 
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