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You won't match star sizes just by picking "matching" filter bandwidths. The cause of magenta halos is that most objects are much brighter in Ha than OIII and SII, so you need to stretch the OIII and SII data more regardless of filter bandwidth. You can fix this during processing in a variety of ways.
For Ha, a 7nm (or 5nm) filter will include NII as well as Ha which can be a good thing.
If you can afford it, a 3nm OIII filter will do the best job of rejecting moonlight.
For SII a 5nm should be fine.
Cheers,
Rick.
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