How to cleanly deal with Chromatic Aberations around stars in astrophotography?

[Nicloks]

Hi All,

Not uniquely an AP problem, but I find I have significant issues removing purple fringing from my photography. I usually stack outside of AP and export the stack as a 16-bit tiff which I then edit in AP.

I find the CA filter does very little and while the Defringe filter does the job, it leaves these dark halo's which I find next to impossible to fix. I've had better success removing purple fringing by doing a color select -> grow -> feather to a mask on a duplicate layer and then using HSL to tweak saturation and luminosity. This still leaves the stars looking bloated which are then troublesome if I am looking to do star reduction work.

Aside from getting better glass for cleaner acquisition, what can I do inside of AP to remove purple fringing without leaving terrible looking artifacts behind?

[NotMyFault]

Hi,

 

could you upload an example document?

Normally you can mitigate halos by playing with the defringe parameters, and play with

• the blend modes, or
• filter opacity
• tweak blend ranges

It really depends on you source image what works best.

In addition, I would never apply the defying globally, but invert the inherent mask and paint in with a soft(!) brush in white where I need the effect.

Or select by colors, create mask from selection, and smooth edges by gaussian blur.

If nothing helps, there are countless advanced methods using selections based on hue, saturation, luminosity or any combination of these.

[Paul Mc]

I've not done this but would selectively applying lens distortion (correction) to the separate channels of R, G and B and then recombining them work?

Here's a page I found which appears to describe a method that could be applied using AP.

https://nightskypix.com/what-is-chromatic-aberration/

 

[Nicloks]

Thanks for the replies. I'll check out that PixInsight link for sure. I've attached a 300% crop of a problem area, first photo prior to any corrections, second with CA and Defringe and finally with the colour selection and HSL layer to desaturate and bump luminance.

[NotMyFault]

I got a good non-destructive workflow:

• convert to LAB/16
• add hsl adjustment
• select blue range
• reduce saturation and lightness 
• to limit the hsl to the CA affected area, use blend range on destination. Select LA channel. Adjust curve until you get the best result.

HSL parameters:

Blend range Parameters:

View of LA channel to see that it can be used for masking:

LA and LB are kind of reverse in the affected area. I tried to use channel mixer, but this had to much collateral damage.

 

[Lisbon]

I would try to replace the fringing with another region of the sky.

On 4/7/2022 at 3:46 PM, Nicloks said:

...it leaves these dark halo's which I find next to impossible to fix...

You can use these dark halos to your advantage. They will allow you to create a mask of the fringing.

Add a defring live filter, make those halos really black and change the blending mode to differece.

Improve the "mask" by adding a B&W adjustment.

Duplicate your background (BG Copy) and right click on any of the RGB channels and choose "Load to Pixel Selection".
Expand the selection (Select > Grow/Shrink) by 1px and add a layer mask to your BG Copy.
Turn off the visibility of B&W adjustment and defringe live filter.

Finally, with the Move Tool, move the BG Copy to the left. Just make sure you have "Lock Children" active, otherwise the mask will move along with the layer.

[NotMyFault]

On 4/7/2022 at 4:46 PM, Nicloks said:

Aside from getting better glass for cleaner acquisition

This self reflection and true analysis is the best course of action. Trying to save an image suffering from inadequate equipment for capturing the source image is a unforgivable waste of time unless you spend it for learning workflows you will never need if you capture the image correctly. Or for forensic use cases.

[Nicloks]

Thanks so much for the detailed responses.

@Lisbon I used your method with really amazing results. I added a bit of feathering and exposure alterations and the black holes left by the Defringe process are all but gone. Only issue is that during further processing there is a slight haloing outside of the Defringe artifact area which become apparent. I'm sure I'll mitigate that with some effort though.

On 4/9/2022 at 5:48 AM, NotMyFault said:

This self reflection and true analysis is the best course of action. Trying to save an image suffering from inadequate equipment for capturing the source image is a unforgivable waste of time unless you spend it for learning workflows you will never need if you capture the image correctly. Or for forensic use cases.

This is true, however economic realities also have to be dealt with. I shoot on older Sony FF cameras and this image was shot with the Samyang 35mm f/1.8 (@f/2.2) Tiny Prime which is $500 where I live. The Sony 35mm f/1.4 GM is probably the best/brightest performing 35mm for astrophotography of any system, but costs $2000. I certainly wouldn't be dealing with CA or other aberrations to nearly this level, however I also wouldn't be eating as there is no way I can afford it.

[NotMyFault]

4 hours ago, Nicloks said:

Samyang 35mm f/1.8 (@f/2.2)

 this explains a lot. I have a dirt cheap Canon EF-50 STM 1.8, and EF-S 24 2.8, too. it is next to useless wide open, but set me back only about 125€ new. And don’t show such strong CA.

There are better lens options for astro. And better Camera bodies. The noise ist extreme, 

[Lisbon]

13 hours ago, Nicloks said:

Thanks so much for the detailed responses.

@Lisbon I used your method with really amazing results. I added a bit of feathering and exposure alterations and the black holes left by the Defringe process are all but gone.

Great!
Glad i could help.

13 hours ago, Nicloks said:

Only issue is that during further processing there is a slight haloing outside of the Defringe artifact area which become apparent. I'm sure I'll mitigate that with some effort though.

Yes. A few tweaks and you should have a clean result.