o.k., I answer my question myself:
Deepsky has some very differing objects. Big dim nebulae, but bright small
objects too. A globular cluster can be observed with high magnification,
it is bright enough and the power is needed to resolve many stars,
and there the colour aberration (allthough not visible as colour!)
of a short achromat will hinder it to show the same contrast than
an equally big apochromat.
Go to a supremely dark site say in the high alpine 6000-7000ft with excellent seeing and do a comparison and see for yourself what the difference if any is on deepsky targets between a perfectly collimated well figured short tube achro and a similar aperture apo at various magnifications and report back the results. Under these conditions a well figured short tube achro can show some pretty amazing detail on deepsky targets when then transparency and seeing are excellent as the quality of the dark skies with not a hint of light pollution.
I'm sure you will notice subtle differences on said targets under similar conditions but it won't be as dramatic as some may think on deepsky objects.
Anyone wanting the answer to this long debated achro/apo question should go out to such a location and view several various deepsky targets at a wide range of magnifications and see for themselves first hand the comparison.
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