How to remove chromatic aberation?

[jnp]

As you all probably know, imaging with a cheap achromatic lens will give you abundant CA especially on a faint DSO needing lots of stretching. This old Pacman image below is poor, (taken using 22 x 240s with a £13 meade 90mm F9 lens + drainpipe), but deliberately chosen for its chromatic aberation.

What's the best way to process out chromatic aberation. I have had a play around with star masks for the first time as per the attached images. I haven't documented all the steps, but I did play with levels of masks where it suited (e.g. reduced blue).

1. Original single layer image - You can see the problem. Large bloated stars with blue fringing.

2. Original merged with a star mask in 'color' mode. At least all the bright stars are not all blue anymore.

3. Original merged with a star mask in 'color burn' mode. The stars are smaller but the dark rings and a blue halos are very obvious.

4 Merged 2 & 3 together 50% each.

I think this could work but I'm sure there must be a better way?

cheers

John

[Skybrowser]

I know we all strive to perfection but the nebula itself is stunning. Can't help you on the CA issue but if I got a shot like that, personally, I'd be well chuffed!

[Ags]

I know nothing about imaging, but... Couldn't you image with RGB filters, refocusing for each color? Then there would be no (or very little) CA.

[jnp]

Couldn't you image with RGB filters, refocusing for each color? Then there would be no (or very little) CA.

I was using a Canon 1000d. I havent used RGB filters. The physics is sound but that sounds quite tricky in practice?

Thanks for the kind feedback, its not bad for a cheapo effort but you should see what the real imagers are doing.

[brianb]

What's the best way to process out chromatic aberation.

Ummm, change the lens for an apochromat ... or use a reflector?

[JonH]

Hi John,

Not sure if you have PhotoShop, but here's a method I use for 'normal' photography -

Copy background, apply Gaussian Blur (found in Filter - Blur) at a radius of about 4 and change the blending mode to Colour. The image stays sharp and the fringing all but disappears.

I usually apply a mask and only 'paint' the blur back in where it's needed because it also affects small areas of red. Obviously not good for astrophotography!

Hope this helps.

Cheers,

Jon