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Greetings from the Northern Hemisphere!
There is a great online article on this topic at http://pages.sbcglobal.net/raycash/filters.htm
Please note the test conditions using a 10" Newtonian. The O-III filter is more restrictive than the Narrowband/UHC/Nebustar/Ultrablock, or whatever other variants they have come up with. When buying one's first nebular filter, the narrowband variants are the logical choice. The O-III filter makes sense as a secondary nebular filter for scopes under 8-10" aperture, but begins to make sense as a single filter strategy with larger scopes.
Clear Skies,
Bob
p.s. -7C and snowing here tonight near Philadelphia
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