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Hi Paul
Minus Violet filters have been around for a while, from several manufacturers for using with achromat refractors. I have several, including the Baader Semi Apo and Williams Optics MV. With my 127mm / f9.4 and 150mm / f6.5 they work well enough , that the colour fringing is not intrusive. Doesn't spoil the view for me and I don't find the colour fringing as problematic as some folks might but it doesn't magically turn the achros' into astrographs and aren't intended too. They are for visual observing and the fringing is really only noticeable on the bright objects.
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