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Star reduction?

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#1 Jeffmar

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Posted 05 December 2021 - 07:47 PM

I have been doing a lot of astro imaging lately. I can tell you that with astrophotography practice may not make perfect but it sure gets you closer. One of the things I have not been getting better at is star reduction. I have tried using star reduction in Photoshop but with mixed results. I was wondering if there is a simple easy to use app without a huge number of steps to do. I have never been one of those people that want to eliminate all the stars around a galaxy or a nebula I am imaging, but every now and then it is nice to tone down a big honking star that is stealing the show. Any help would be awesome.

 

The Horsehead and Flame Nebulae
 
Did you notice the big honking star?

 



#2 vidrazor

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Posted 05 December 2021 - 09:35 PM

How big is a star in a single sub? Most star bloat happens in post. I don't know how much you can shrink something that big, but in Photoshop you can shrink by way of Starnet++. Make a starless render of your final render and place it in sync on top of you final render, and change the Starnet++ layer from NORMAL to LIGHTEN. Place a levels adjustment layer between them, and move the Gamma (midpoint) control right to darken midpoint data on the base (full image) layer. This will reduce star size, and the slider will control the reduction, and the Starnet++ layer will maintain target and sky density. You will probably need to try something different for really large stars. Annie's Astro Actions has some star reduction processes for small and large stars you can try as well for $20.


Edited by vidrazor, 05 December 2021 - 09:37 PM.

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#3 17.5Dob

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Posted 05 December 2021 - 09:38 PM

The best way is to keep from blowing them out in the first place. All of your brighter stars are severely oversaturated from too high an ISO, too long an exposure, or clipping during post processing. At that point there is nothing you can do.....

I did no star reduction in this image. This is just the normal result of using PI's masked stretch if your stars are reasonable exposed to begin with.


Edited by 17.5Dob, 05 December 2021 - 09:42 PM.

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#4 Jeffmar

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Posted 05 December 2021 - 10:43 PM

The best way is to keep from blowing them out in the first place. All of your brighter stars are severely oversaturated from too high an ISO, too long an exposure, or clipping during post processing. At that point there is nothing you can do.....

I did no star reduction in this image. This is just the normal result of using PI's masked stretch if your stars are reasonable exposed to begin with.

That is an impressive image! Can you tell me what your exposure lengths were, and the ISO or equivalent you used? 

Every bit of information is helpful in this hobby!


Edited by Jeffmar, 05 December 2021 - 11:16 PM.


#5 erictheastrojunkie

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Posted 06 December 2021 - 01:31 PM

 

I have been doing a lot of astro imaging lately. I can tell you that with astrophotography practice may not make perfect but it sure gets you closer. One of the things I have not been getting better at is star reduction. I have tried using star reduction in Photoshop but with mixed results. I was wondering if there is a simple easy to use app without a huge number of steps to do. I have never been one of those people that want to eliminate all the stars around a galaxy or a nebula I am imaging, but every now and then it is nice to tone down a big honking star that is stealing the show. Any help would be awesome.

 

 
 
Did you notice the big honking star?

 

In the case of Alnitak, well, you're pretty much screwed. Like Dob said, the best way is to keep the star from getting big in the first place and that starts with acquisition. Getting high quality subs stacked with very low FWHM values is the first thing that has to happen when you have a big star that dominates an image or a target that is swamped in a star field (think Veil nebula). Then in post processing doing careful stretching, like a masked stretch, keeps the stars from getting huge. The last thing you should attempt is star size reduction, but on a star like Alnitak that just won't work. 


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#6 BQ Octantis

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Posted 06 December 2021 - 01:52 PM

Hi Jeff,

 

A big part of the problem is your color balance + stretch method exacerbated by a lack of flats. There is virtually no data in the periphery, so no room to maneuver.

 

Alnitak is especially a PITA in the blue, but you can use the red and green channels to tame it, as I described in several posts in this thread.

 

Post a flat and a light and we can see just how far down this rabbit hole we can go…

 

BQ


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#7 vidrazor

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Posted 06 December 2021 - 06:56 PM

Hi Jeff,

A big part of the problem is your color balance + stretch method exacerbated by a lack of flats. There is virtually no data in the periphery, so no room to maneuver.

Alnitak is especially a PITA in the blue, but you can use the red and green channels to tame it, as I described in several posts in this thread.

Post a flat and a light and we can see just how far down this rabbit hole we can go…

BQ

Adding to the borrowing from the Peter channel to pay the Paul channel, luminosity and contrast mods and color neutralization helps a lot to tame the beast. I added dehaze, contrast reduction, multiplication and color neutralization to the equation in the image over there, the image here I feel is too far gone for that. I think HDR is the only real ways to deal targets like this or M45.

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#8 Jeffmar

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Posted 06 December 2021 - 07:18 PM

Adding to the borrowing from the Peter channel to pay the Paul channel, luminosity and contrast mods and color neutralization helps a lot to tame the beast. I added dehaze, contrast reduction, multiplication and color neutralization to the equation in the image over there, the image here I feel is too far gone for that. I think HDR is the only real ways to deal targets like this or M45.

That is some good looking post processing. I would be pretty happy with that bottom image. Thanks for your help!



#9 BQ Octantis

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Posted 06 December 2021 - 07:19 PM

…the image here I feel is too far gone for that…

 

Only because it's been stretched almost to shadow clipping at the periphery (because of no flats) and to highlights clipping in the blue channel (because of poor starting color balance). But if you look at the red channel, there are heaps of details up close to Alnitak.

 

If the OP would post just a single raw light and a single raw flat, we can see if we can apply some of the "borrowed" techniques.

 

BQ




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