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Wrt Clive's comment... Astrodon makes a red continuum filter. It's basically a 5nm Ha filter with the centre bandwidth shifted slightly (645nm instead of 656.3nm). The idea is that you can subtract a continuum sub from a Ha sub and broadband sources like stars are removed without affecting the nebulosity.
Like all other star removal methods the results are not perfect.
Cheers,
Rick.
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