Light pollution is broadband: streetlights, skyglow and moonlight smear energy across the whole spectrum. Emission nebulae are the opposite: ionized gas radiates at a handful of exact wavelengths — hydrogen-alpha at 656.3 nm, doubly-ionized oxygen at 500.7 nm, sulfur-II at 672.4 nm. A narrowband filter is a gate that passes a few nanometers around one of those lines and rejects everything else. The nebula walks through untouched; the city loses by a factor of a hundred. That asymmetry is the entire trick, and it's why a Bortle 8 balcony can produce Ha data that competes with dark-site broadband.
The same gate defeats the moon. Moonlight is reflected sunlight — broadband — so a 3–7 nm Ha filter keeps working within reasonable distance of a bright moon. This single fact roughly doubles your usable nights per month, which is a bigger upgrade than almost any optic.
The catch: it only works on emitters
Narrowband does nothing for galaxies, reflection nebulae, or star clusters — their light is broadband, so the filter throws it away along with the light pollution. There is no filter that makes a galaxy punch through a city sky; 'light pollution suppression' filters for broadband targets earn their living in the margins at best. For broadband work the only real answers are darker skies, more hours, and gradient-handling in processing.
For one-shot-color cameras the practical tool is the dual-band filter (Ha + OIII passed simultaneously, e.g. L-eXtreme/L-Ultimate class): one filter, one stack, both lines. It's how OSC imagers shoot nebulae from cities. Mono cameras take the full path — separate Ha/OIII/SII filters, combined into the Hubble-style SHO palettes — with more control and more total effort.
- First narrowband filter: Ha, always. Strongest line, most targets, works under the worst moon.
- Narrowband flats take much longer to expose than broadband flats. That's normal — don't fight the panel, just let them run.
- Filter bandwidth (7 nm vs 3 nm) buys contrast at the price of exposure time and cost. 3 nm shines at f-ratios slower than ~f/3 and under serious light pollution.