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Recommended:
Astronomik UHC visual
TeleVue BandMate II Nebustar
Lumicon UHC Gen.3
DGM NPB
ICS UHC
Recommended only if you can test the individual filter:
Orion UltraBlock
Thousand Oaks LP-2
Nebulae emit light in discrete wavelengths and any other light passing the filter is extraneous to seeing the nebula.
The more of the extraneous light suppressed, the darker the background behind the nebula and the more contrast there is between nebula and sky.
To achieve that, a bandwidth in the filter that just passes the light of the nebula is ideal.
But there are always slight irregularities in the production of filters, and they react differently to short f/ratios like f/4 or faster, so the bandwidth needs to be about 8nm wider than the minimum possible bandwidth of 14.6nm (the spacing between the H-ß line and the O-III lines)
So a 23nm filter would be the ideal.
Most of today's higher-quality UHC type narrowband filters are in the 22-27nm bandwidth range, like Astronomik, TeleVue, DGM, Lumicon, ICS.
45-50nm is very common among the Chinese UHC-type filters, so you want to avoid these if you can.
So since only 22-23nm of bandwidth is necessary for maximum contrast, the extra bandwidth just lets a bit too much light pollution through (why it only works best in a truly dark sky).
There is a little contrast enhancement, just not enough to make a large difference.
So, what can you do to maximize the experience?
1. Use only low powers with the filter. On an 10" scope, a maximum of 100x.
2. Make sure you are as dark-adapted as possible. That means 30-45 minutes outside, away from all lights, i.e. at least 30 minutes after turning off all lights.
3. Make sure the object you're looking at is at least 30° above the horizon. Your nebula target would lose ~0.25-0.3 magnitudes of brightness at the zenith, and double that at 30°.
It could lose a whopping 2.5-3.0 magnitudes at the horizon!! So try to view the nebula when it is near or crossing the imaginary N-S meridian.
4. Make sure the nebula is an emission-type nebula. The nebula filters won't help reflection nebulae like those in the Pleaides or M78 in Orion.
5. The 2" size is more universal--it fits 2" eyepieces, the bottom of most 2" to 1.25" adapters, and 2" star diagonals in compound scopes.
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