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Note on armchair astrophysics:
Combining the H-alpha and SII channels to create a third channel which one colours blue is quite legal artistically.
Scott is speaking artistically when he mentions a "pseudo-OIII".
However, for the folks who aren't sure, you cannot really infer the amount of OIII from the amounts of H-alpha and SII.
If you know the latitude and longitude of a boat, you can't reliably work out from that how much fuel it has.
The whole idea of three very expensive narrowband filters is that they provide three quite independent pieces of information, and you can't (in general) work out the third from the first two.
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