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Just thinking about what we'd expect to happen. Not absolutely sure of my ground and I might be wrong on the fine detail.
Space telescopes can see far infrared, in order to see straight though thick dust, and see T-Tauri stars, protostars and proto-planetary discs, but need to be cooled with liquid nitrogen or helium, and need very special detectors. That's not relevant to us.
A near infrared filter with a cut-off at say 700 nM will let through near infrared from stars, while blocking emission from Ha, OIII, SII, NII, CN, etc.
Consequently, if our camera is sufficiently sensitive to near infrared, we might hope to see ordinary stars while blocking out emission nebulosity.
If we're seeing nebulosity with a near infrared filter with a gentle cut-off so it is 50% down at 650, it is probably H-alpha or NII which has just snuck past the IR filter.
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