Narrowband vs Broadband: Choosing Filters for Your Sky

Filter choice is really a question about your sky, not about your camera.

Broadband: LRGB

Broadband filters pass the whole visible spectrum in bands. They are the only way to record true star colour, and the only sensible option for galaxies and reflection nebulae, which emit across the spectrum. They also pass light pollution, so they perform best from darker sites.

Narrowband: Ha, OIII, SII

Narrowband filters pass a few nanometres around a single emission line. A 7nm Ha filter ignores most of a city's light and most of the moon's contribution, which is why narrowband imaging works on nights when broadband imaging does not.

Dualband

A dualband filter passes Ha and OIII together, usually in one 10 to 12nm window each. It is the single most useful filter for an unmodified colour camera under light-polluted skies, because it recovers both channels in one sequence. It will not give you SII, and it will not give you true star colour.

Matching filter to target

Emission nebulae reward Ha and OIII. Planetary nebulae often show strong OIII. Wolf-Rayet shells are dominated by Ha with a weaker OIII halo. Galaxies and clusters want broadband and nothing else.

A note on bandpass width

Narrower is not automatically better. Very narrow filters reject more light pollution and more moonlight, but they also need longer sub-exposures to get above read noise. A 6 to 7nm Ha filter is a sensible default for most amateur rigs.

If you are unsure where to start, tell the Filter Advisor which filters you own and what you want to image, and it will match them to the target.