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Hi Aidan,
I am not familiar with the other objects you have posted but I do have a side by side comparison of an easy target at the moment: NGC2437 (M46).
The star cluster includes a planetary nebula that is strong in Ha and weak in S2. See the attached screen cap for a two uncalibrated frames linearly stretched the same from the bgd, both about 1297 adu, to 2000 adu, which is just above the brightest part of the ring. The very brightest part of the ring is about 100 adu for S2 vs ~700 adu for Ha above the bgd.
This is from a Apogee U16M (16803 CCD), what CCD do you have?
HTH,
EB
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