Filters choose which light reaches the sensor. LRGB (luminance + red/green/blue) capture natural color with mono cameras. Narrowband filters (Hα 656nm, OIII 501nm, SII 672nm; 3–7nm wide) isolate single nebula emission lines and reject nearly all artificial light. Dual-narrowband filters (L-eXtreme, L-Ultimate) pass Hα+OIII together for one-shot-color cameras — the simplest way to image emission nebulae from a city. For light-polluted skies a dual-narrowband filter is a game-changer.