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Thanks Jeff and Greg for the replies.
I found a thread which suggested I have the filters the wrong way round according to Chroma (they are AR coated on only one side), but the right way round according to the consensus in that thread!
I've flipped them anyway and will await a clear sky to test ... may be some time!
Still not understanding why Ha showed a halo and OIII not. That star is a broadband emitter. Slightly red so a bit more in the Ha bandpass than in the OIII, but still should be plenty of light bouncing round in the OIII case.
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