I'm not fond of the Canon EF 200mm f/2.8L II USM telephoto fixed lens. I find I'm in a constant battle in an attempt to control chromatic aberrations. In this photo of Andromeda the use of a 1.4x tele-extender only makes the aberrations worse. I used Photoshop to desaturate the purple rings around stars. I have tried several other techniques.
Here I used an old trick of visual astronomers who used achromatic telescopes. I defocused the lens to make the different colors fuzzy so they blend in together.
Here I separated the RGB layers, and then restacked the colors with slightly different magnifications.
Here I converted to single channel B&W.
Here I do a tight crop so that there is less lateral chromatic aberration visible. The Canon EF 200mm f/2.8 lens' small size, light weight, moderate focal length, fast focal ratio, great build quality, and thoughtful ergonomics makes this my most used lens/telescope. Too bad it is not very good optically.
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