Optical ones have to be specifically tailored to the lens. I was referring to software. You do lose something in software but the results are better than without. I know you can do stacking in photoshop but never tried this kind of stacking. Rather than play with another SW package I may try tinkering with the capabilities in what I have and see if I can get similar results.
I didn't want to lose any light since I couldn't use longer exposures on this occasion. I can't remember if the 135/2 did improve much stopped down now... and when you say CA filter, do you mean the optical sort or software sort? Effectively I did my own version of a software approach for this image. I haven't looked at exactly how the optical ones work but presume they filter out certain wavelengths so as to reduce the effect. But I'm not sure they're applicable in this scenario anyway.