I have difficulty seeing nebulae such as the Blinking Planetary, NGC 6826 or the N. American Nebula, NGC 7000 (neither of which I've successively observed) and I do not see others well, such as the Clown Face, NCG 2392 even from a nearby dark-sky site (~Bortle 2.5) near Granite Mtn. outside of Prescott, AZ.
I have a cheap filter, Svbony UHC, O-III but notice little to no improvement, just a little coloring. Would it be worthwhile to buy a more expensive filter such as the Lumicon UHC-filter or at least the Optolong UHC Nebula Filter?
And speaking of filters, what about one or a couple planetary filters to increase contrast, such as a light blue #82A or others?
Emission nebulae emit light in the spectral lines of Hydrogen and oxygen. The lines that are relevant for our night vision's sensitivity are H-ß and O-III
Unless you know the spectrum of the nebula, a filter that passes both H-ß and O-III will be more "universal" than one that passes only H-ß or only O-III.
In general, a narrower bandwidth to the filter rejects more sky light and enhances contrast more between the nebula and sky than a filter with a wider bandwidth.
The ones with nice narrowband bandwidths that pick up those 3 spectral lines but enhance well are TeleVue Nebustar, Astronomik UHC, DGM NPB, Lumicon UHC, ICS UHC
Many less expensive filters have wide bandwidths and enhance a lot less.
Tricks for use to achieve maximum results with a filter:
--use low power, under 10x/inch of aperture. Higher powers don't work as well with the filters because contrast is significantly reduced.
--you must be well dark-adapted. That means at least 30-45 minutes outside away from all lights. If you are not dark adapted, the filters will be worthless to you.
--try to observe the nebulae above a 30° altitude in the sky, preferably when the nebula crosses the N-S meridian in the sky. The higher the nebula, the easier it is to see.
--make sure the nebula is one that can be seen in a scope, not just one that is typically photographed. Don't even bother with the Heart and Soul Nebulae, the California, etc. unless you are viewing from a pristine sky with no light pollution.
--be aware the overall field will be darker because the light of the stars is suppressed along with sky light. The nebula filters are for looking at nebulae, not stars.