Dealing with Chromatic Aberration in DSLR lenses

[themos]

A lot of us are shooting widefield tracked DSLR shots with a 135mm lens or similar. If you have a cheap (tens of £££, not hundreds) lens you will notice that even when you stop down the lens to get round stars over the whole frame you still end up with nasty red halos like the first pic. These things appear even though you've done your best to focus properly. So, what can you do about it? Unfortunately there's no satisfactory software solution, otherwise people would not spend £100s on APO lenses. But there is a way round it if you are prepared to spend some extra time imaging.

I've noticed that "ordinary" focusing seems to bring the green and the blue colours to a nice focus and it's usually the red that is off by a bit. That's what you can expect from doublets optimised for daytime photography as blue and green edges are everywhere (foliage and sky, right?) but red edges are rare in nature. So, keep your green and blue signal (that's 3 out of 4 pixels under a Bayer matrix) from your ordinarily focused shots but get your red signal (1 out of 4 pixels) from a different focused shot.

To get my red signal, I used a red filter in front of a 135mm lens and LiveView focused using that on a bright star. Of course, the green and blue is all over the place but we don't care about that in this case. That's the second pic. The problem is that we are wasting 75% of the light but that's the price of a cheap lens.

Once you have the frames, you throw them to DeepSkyStacker or similar. I used super-pixel mode so all RGGB pixels will end up in the same colour pixel and the resolution will be halved. You process the blue-green focused frames separately from the red-focused ones. I ended up with a BG.tiff and a R.tiff (but they are both full colour images). In IRIS, you use SPLIT_RGB command to get the blue and the green channels from BG.tiff and the red channel from R.tiff. You then re-register the red channel onto the blue or green channel with the Stellar Registration menu command and then use TRICHRO command to combine the registered Red channel from R.tiff with the G/B channels from the BG.tiff. It works quite well. The animation shows crops from the red channel of the different focused stacked .tiffs. The North America Nebula shot is one that was processed in this fashion.