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Three Techniques for CA Reduction in Photoshop Compared

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#1 BQ Octantis

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Posted 21 February 2019 - 09:53 PM

G'day all,

 

I've been experimenting with axial chromatic reduction in Photoshop again. I've been using the green channel as a darken layer to moderate success for some time now. But it requires careful gamma adjustment to find the point where I take out star CA without affecting other colors in the image (reds in particular). And it invariably leaves ugly black circles around the stars in place of the CA ring.

 

Someone put me onto the Reduce Blue/Purple Halos action in Astronomy Tools v1.6, but its results are quite similar to my green layer method. I've been using Annie's Astro actions for the last few months; while it has no CA removal tool, it suddenly dawned on me this week that I could use its Remove Stars function to remove CA without impacting any reds in the image—in just four simple steps:

  1. Duplicate Layer
  2. Play action "Remove Stars"
  3. Set layer mode to "Color"
  4. Merge layer with background

Here are the three methods compared on some stars near the Running Man nebula:

 

fringe_killing.jpg

 

The result is a white ring around the star, which is then simply part of the star bloat (and reducible as such).

Cheers,
BQ


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#2 james7ca

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Posted 22 February 2019 - 01:13 AM

Thanks for the tip.

 

Or, you can do a simple Noise Reduction filter in Photoshop with a large boost to the color noise removal. This technique works really well when you have a color bias on very small stars. The below example was done without a mask, but if you want to avoid any effect on the background you can just use a star mask with the noise reduction filter.

Attached Thumbnails

  • Color Noise Reduction.jpg

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#3 Marco1968

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Posted 22 February 2019 - 03:56 PM

Greetings BQ,

I have used with some success the lens correction sliders in Adobe Camera Raw that comes with Photoshop (can be downloaded for free as a plugin).

You may give ir a try: look for sliders in Adobe Camera Raw that allow correction of chromatic aberration, specifically reducing the purple ring.

What ACR does is desarurate the purple ring allowing then an easy star size reduction.

The deep sky background is not affected at all.

Hope this is interesting for you, as they say YMMV

Clear Skies!

Marco

#4 BQ Octantis

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Posted 23 February 2019 - 11:31 PM

Thanks for the tip.

 

Or, you can do a simple Noise Reduction filter in Photoshop with a large boost to the color noise removal. This technique works really well when you have a color bias on very small stars. The below example was done without a mask, but if you want to avoid any effect on the background you can just use a star mask with the noise reduction filter.

Wow!…I LOVE this method. It never dawned on me to use the PS color noise filter. It leaves enough residual color to produce pleasing results with a star color increase. And no ugly circles. And you can reapply on subsequent processings.

 

Like all methods, it attacks nebula details—somewhere between the green layer method and the Annie's method. Fortunately, I found a convenient way to select the bright stars with CA with Astronomy Tools v1.6. If you use the Select Brighter Stars action, it'll select all stars; you then contract the selection by 2 pixels (which deselects all the smaller stars). If you then regrow the selection by 2 pixels and convert it to paths with the selection-to-path tool, you can then attack just the CA rings with a stroke path with a 13-pixel 0% hardness brush on the layer mask.

 

Every once in a while, I come across a game-changing method that makes me want to reprocess all of my images. This is most certainly one of those. Here's the method on my Carina Nebula shot—the most challenging of all my shots for CA reduction:

 

get.jpg?insecure

 

Cheers!

BQ



#5 BQ Octantis

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Posted 23 February 2019 - 11:36 PM

Greetings BQ,

I have used with some success the lens correction sliders in Adobe Camera Raw that comes with Photoshop (can be downloaded for free as a plugin).

You may give ir a try: look for sliders in Adobe Camera Raw that allow correction of chromatic aberration, specifically reducing the purple ring.

What ACR does is desarurate the purple ring allowing then an easy star size reduction.

The deep sky background is not affected at all.

Hope this is interesting for you, as they say YMMV

Clear Skies!

Marco

 

Thanks, Marco. My Adobe RAW processor just has a Defringe pulldown menu with three options:

 

Screen Shot 2019-02-24 at 2.02.58 PM.png

 

The All Edges option seems to reduce the CA significantly. I'll have to try it on my next stack…

 

BQ



#6 calypsob

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Posted 25 February 2019 - 03:02 PM

Thanks, Marco. My Adobe RAW processor just has a Defringe pulldown menu with three options:

 

attachicon.gif Screen Shot 2019-02-24 at 2.02.58 PM.png

 

The All Edges option seems to reduce the CA significantly. I'll have to try it on my next stack…

 

BQ

I strongly recommend upgrading to PS CC, the new Camera Raw chromatic aberration tool is much more powerful.

Often times you will find that chromatic aberration has shifted a color out of its white balance, stars abberating to red/magenta are typically oversaturated yellow stars. Purple stars can actually be Cyan or blue stars, don't always default to bleaching out the stars trying to make them white, its good too look at some reference images or even a star catalog.

 

This is where the newer PS CC becomes a powerful tool, you can use camera raw as and effect layer. If you make a highlight mask of your image and make the layer a color layer, you can focus on chromatic abberation alone, and even shift the hues or use selective color to manually white balance the stars independent to the background.

 

I found a table on this forum that does a good job of showing how colors tend to shift when they abberate. Of course this is entirely dependent on the optics you are using, some may not have abberations at all.

I know from processing data out of the canon 200mm f2.8 that the blues can tend to shift into the purple spectrum and the yellows tend to shift into the magenta/red spectrum.

 

https://reshade.me/f...atic-aberration


Edited by calypsob, 25 February 2019 - 03:04 PM.


#7 james7ca

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Posted 25 February 2019 - 10:57 PM

Lateral chromatic aberration is fairly easy to remove, however, what the OP is showing looks like axial or longitudinal chromatic aberration which is much harder to correct. Photoshop can do a good job on lateral chromatic aberration, somewhat less well on other types of color aberration. That said, Photoshop can do wonders to correct for many imaging problems.



#8 BQ Octantis

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Posted 10 March 2019 - 07:35 AM

I have used with some success the lens correction sliders in Adobe Camera Raw that comes with Photoshop (can be downloaded for free as a plugin).

The All Edges option seems to reduce the CA significantly. I'll have to try it on my next stack…

Marco,

 

I was able to compare the RAW CA processor functions with the CS5 Reduce Noise filter. The Highlights Only function barely reduces the CA, but the All Edges function does a decent job of reducing CA without impacting the nuanced colors in the image. In comparison, the Reduce Noise function does a beautiful job of preserving star colors, but it damages some of the nuanced colors in the nebula. Here are the three methods applied to the core of the Carina Nebula (along with no CA reduction—see the red streak below the Keyhole Nebula in particular):

 

compare.jpg

 

All things being equal, the RAW processor is MUCH faster than a by-sub noise reduction in the workflow—it's just tossed in as part of loading the image. The All Edges function is thus now my go-to.

 

Cheers!

BQ




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