Well,
1) the most useful nebula filter is a narrowband.
It should pass the 486.1nm H-ß line and the 495.9 and 500.7nm O-III lines at >90%.
It should have a bandwidth (Full-width, half maximum, or FWHM) of 22-27nm for best results.
Without knowing the spectrum of the nebula, it will work for:
--planetary nebulae
--large emission nebulae with star forming regions, like M42/43, M8, M17, M20, M16, NGC7000, etc.
--Wolf Rayet excitation nebulae, like NGC6888, or NGC2359
--large emission nebulae without star formation, like IC434, et.al.
--supernova remnants like M1, or the Veil nebula
2) the next most useful filter is an O-III filter.
It should pass BOTH the 495.9nm and 500.7nm O-III lines at >90%
It should have a FWHM bandwidth of 11-13nm for best results.
It will work best for planetary nebulae, supernova remnants and Wolf-Rayet excitation nebulae.
3) the least useful nebula filter is an H-ß filter.
It should pass the 486.1nm H-ß line at >90% and have a bandwidth (for visual use) of 6.5-9nm.
It will work best on the large hydrogen emission gas clouds that are low surface brightness, many without star formation.
It is not the best filter for most nebula, and its more limited use is why I rank it 3rd.
Nebulae not helped by a filter:
--reflection nebulae (exception: when associated with a large emission nebula)
--dark nebulae. This requires excellent contrast found in dark skies.
And, of course, no other object is really helped much by a filter.
The one exception is a very wide O-III filter that has a bandwidth that extends upward to cover the 511nm and 514nm C2 lines found in comet tails.
An example of one of these: Omegon O-III filter, with a 25nm bandwidth that extends up to 517.5nm.
I wouldn't recommend it as your only O-III filter, though.
Edited by Starman1, 16 January 2025 - 06:58 PM.