Thanks Shaun!
One note about nebula filters -
They will not work with galaxies! in the same way as they do with nebulae.
Galaxies, like stars, glow across the entire spectrum. As a percentage of their total light output, the OIII and Hydrogen spectrums form bugger all, and so using these filters with galaxies will only kill them stone dead.
HOWEVER (there's always a "but"...), if your scope is BIG, over 14" in aperture, with a handful of galaxies the OIII and UHC filters are useful in showing up the BIG HII regions in some galaxies.
Two galaxies in particular are prime targets for this, M33 and M83. These two large face-on spiral galaxies have enormous glowing Hydrogen gas regions within their arms. A nebula filter can help reveal these strings of enormous star forming regions within these distant island universes. But, spotting these is an huge test on the quality of transparency of your DARK sky. Poor transparency will kill any attempt at seeing these difficult-to-spot regions, and poor transparency will also kill attempts to see galaxies at all. Maximizing your chances at seeing galaxies, nebulae and other DSO's is not just about "going bush", but the location where you set up is also vitally important. But this is another topic altogether.
I've so far made just one attempt as trying to see these regions in M83, but the location I was at, despite being dark, is plagued by poor transparency. I had no chance at seeing these Hydrogen regions despite the 17.5" scope I was using. I haven't given up and I now use another site that has much better transparency, and despite it being closer to Sydney, its location is such that the sky quality here makes the other site look like a foggy cemetery. The season for tackling M33 has started, so I'll be having a crack at it too. The Large Magellanic Cloud and Small Magellanic Cloud also have extensive glowing Hydrogen regions that benefit from using a nebula filter to search them out. M31, the Andromeda Galaxy, does not have vast enough Hydrogen regions to make them stand out with a nebula filter. Wait a billion years or two when our Milky Way galaxy and M31 begin their merry gravitational dance into merging, and
then massive star forming regions will be triggered within M31!
I've mentioned galaxies here for two reasons: one they are not targets to try out nebula filters with; and two, that there IS a place for nebula filters with galaxies in trying to spot nebulous regions within them - you are NOT making the galaxies stand out here, just those glowing Hydrogen regions.
Alex.