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I have a 120mm skywatcher 600mm fl achro as my guide scope and have tried to image through it with coloured filters and my mono CCD.
The red and green filters give lovely sharp images with pinpoint stars. The blue filter gives stars with big halos around them. I fiddled with the focusing for ages and still the halos remain. I don't have a minus violet filter to try but it might be worth giving it a go stacked on the blue filter.
Narrow pass filters would be a different option and shouldn't suffer from the same problem.
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