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Filters for scopes
Here is the low-down on filters.
Broadband filters--reduce wavelengths at which some lighting broadcasts. Makes the most difference in already-dark skies. Enhancement is minimal, kind of like turning up the contrast a tiny bit. Sometimes these are called light pollution reduction filters. Not really worth owning unless you already observe in dark skies and this would be your 4th choice. Good examples: Lumicon Deep-Sky, Orion Sky Glow filter, Thousand Oaks LP-1
Narrowband filters--Also called UHC filters, these filters pass the wavelengths at which emission nebulae emit: 486nm blue (Hydrogen Beta), 496nm blue green (Oxygen III) and 501nm blue green (O-III, 2nd line). This is the "universal filter" and transmits all the wavelengths from every nebula that emits brightly. You don't have to guess. Not effective on bluish reflection nebulae (these are full-spectrum and need darks skies, not filtration) or dark nebula (ditto previous comment). Good examples: Lumicon UHC, DGM NPB, Thgousand Oaks LP-2, Orion Ultrablock
Line filters-- represented by O-III filters (most effective on most planetary nebulae and certain emission nebulae) and H-Beta filters (work well when the primary emission is hydrogen, like IC434 behind the Horsehead nebula). These filters are specialty filters and create even more contrast enhancement for the objects they are best used for. Your 2nd choice filter (after a narrowband) would be an O-III, followed distantly by an H-Beta.
Good companies: Lumicon, Thousand Oaks, Orion.
The above information and David Knisely's article will tell you what you need to know.
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