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I have assumed/concluded that the 12nm has a twice as wide bandpass than the 6nm - which would catch more Ha, yes ?
No it doesn't work like that, Ha is at precisely 656.3nm wavelength, there is no Ha outside that frequency. However NII is a doublet at 654.8nm and 658.4nm.
So the wider bandwidth Ha filters will allow the NII to pass through as well. You would need a 3nm Astrodon filter to pass Ha only and reject NII.
Cheers
Bill
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