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Nebula filters
What differentiates one nebula filter brand from another is bandwidth. The narrower the bandwidth, the better the contrast--to a point. If the bandwidth is so narrow one of the intended wavelengths is partially blocked, then the filter is less effective.
If you buy only one filter, it would be a UHC-type, which passes the O-III and H-Beta lines in the spectrum. Some nebulae have a lot of emission at one or the other, and would be better-enhanced by a filter that just transmits that wavelength (example H-Beta only), but a good general use filter for nebulae is the UHC-type (UHC was a name coined by Lumicon, and it stuck).
Broadband, or LPR filters, are jus too wide to really enhance nebulae much, though they do seem to turn up the contrast a bit if you are already in dark skies. I wouldn't use one anywhere near a metropolitan area, though, as they don't block enough light.
Something to look for:
--transmission of over 90% at the correct wavelengths (486nm for H-Beta, 496nm and 501nm for O-III and all 3 for a UHC)
--near square-wave response curves
--no reports of bad threads that don't fit eyepieces
--very little response outside the correct wavelengths--preferably zero outside of the chosen wavelengths from 450nm to 700nm.
The spectrophotometer at work confirms a reported analysis of the DGM NPB. It is slightly narrower than the Lumicon in bandwidth.
So we have some choices from narrow bandwidth to wide (remember, narrower produces better contrast):
DGM NPB(the narrowest of the UHCs)
Lumicon UHC (a little wider)
Thousand Oaks Type 2
Orion Ultrablock
Astronomik UHC (one of the widest UHC filters)
Baader UHC-S (a narrow broadband, if you will)
Lumicon Deep Sky (a broadband filter)
I think TeleVue and Baader UHC filters have too-wide bandwidths (the TeleVue O-III filter is wider than the Lumicon UHC, for example), though they may provide pleasing images in small scopes if you know in advance that they will not provide the best contrast.
Hope that gives you an insight into how filters enhance nebulae. Since stars are full-spectrum objects, their lights are dimmed by good filters, but the nebulae aren't. You're turning up the contrast quite a bit, and that enables the really faint parts to stand out.
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