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I think it's actually there Greg. If I recall someone telling me once it's H beta, close to Ha that turns up green in the hubble palette. Same thing is at the tip of the HH in similar NB pics.[/QUOTE]
Oh yes I realise its there - its a strong Ha area in the image as Ha is assigned to the green channel in NB Hubble palette. Problem in NB Hubble is the Ha signal is many times stronger than O111 and especially S11 hence the tendency for the image to be predominantly green with some blue areas. The challenge I think in NB is to tame this green dominant phenomena with a more balanced palette. Russell Croman is probably the king of NB and so Ken Crawford the prince so examples of their work make a great template for a very pleasing colour balance (Russell's in particular, a 20inch RCOS and New Mexico skies no doubt help!).
I notice Astrodon is selling N11 (nitrogen11) filters. They seem to give a strong image on supernova remnants etc. Could be a nice alternative to S11 which is a bit of a pain really. There's also a helium filter but not sure how strong that signal is. You can also spin a polarising filter (I have one but never used it) through various degrees (the side of the filter is marked) 45 90 120 270 degrees to give 4 different exposures and then assign them to colour channels in photoshop to get a polarised colour image. It would work on Eta Carina, M1.
Another great image by the way. You pump them out with that setup Mike to a consistently high standard.
Greg.
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