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Hi John,
Ive seen some good shots through some 120mm refractors with minus violet filters. A couple of cheaper refractors though stars were sharp in one spot and not others.
I have a vixen 140 neoachromat as they have a pretty flat field with sharp star images across. I have posted a quick snap of eta-carina previously with a Baader UHCS filter. This seems to eliminate pretty much all fringing for me. You can reduce the green channel a bit to come up with a fairly pleasing image.
Recently I've obtained a semi-apo filter which is very good for visual - lots of light throughput and dramatic reduction (but there is not complete elimination) of the purple finging.
I think the GSO 8f4 would be a great option too with a coma corrector. I got the neoachromat to experiment with for a bit for fun. It is a good telescope for deep sky for its aperture but still trying to sharpen it up a bit on planets. I was considering the GSO (arguably a better option).
Have you tried a focal reducer with the VC200L - an great unit.
There have been some good (amazing in fact) images through achromats using the HDR method in the ice in space deep sky photography forums.
Cheers
David
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