Capturing nebulosity requires low focal ratio. To get optimal correction of chromatic aberration and coma at low focal ratios requires more lens elements. Yes you can use an ED (or similar) doublet with a coma corrector, but that’s 4 lens elements in total, and in most cases it won’t be as optimal as it could be, and there’s likely to be spacing errors, so better really, if you know the telescope will be used for imaging at this focal ratio, to build it all into one optimised, correctly measured-up tube assembly. Many people wish to use the dual-band, tri-band and quad-band filters for one-shot imaging, so chromatic aberration will still be a moot point.
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