>> ...I would need of course a red filter >> that excludes hydrogen alpha. Not true. See any of the large number of papers in the professional literature that obtain H-alpha images just in the manner I described. The problem is that the H-alpha filter is not so narrow that it excludes the red continuum. So you subtract out of the H-alpha image some fraction of the flux using the broadband red image. \Brian -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.