Nebula filter
Also called: UHC, OIII, O-III, H-beta, Hβ, narrowband filter, LPR filter, light pollution filter.
A filter that passes the light emitted by nebulae and blocks most skyglow, making them stand out.
Emission nebulae, planetaries and supernova remnants shine in a few narrow colours. A UHC (narrowband) filter passes hydrogen-beta and oxygen-III light; an OIII filter passes only oxygen — superb on planetaries and the Veil; an H-beta filter is the key to a few objects like the Horsehead and California Nebula.
Galaxies, star clusters and reflection nebulae shine with ordinary starlight: nebula filters only dim them.
At the eyepiece
Filters work best with a fairly large exit pupil (3–6 mm).
In AstroPilot
Object pages say which filter suits each object, and whether you own one.