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The 3 nm also rejects the NII signal too, but it's picked up by the 5 nm. The NII signal can be substantial depending upon the object (e.g. some of Bert's NII images).
I emailed Don Goldman a while back before ordering my 3 nm Ha filter. He recommended either (1) 5 nm Ha & SII + 3 nm OIII, or (2) all in 3 nm. Scenario 1 was recommended if cost was an issue, or if picking up NII with the Ha was important. Scenario 2 was recommended for imaging under heavy light pollution. OIII is more susceptible to light pollution than Ha or SII (especially moonlight), so he recommended getting the 3 nm regardless of the other two.
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