Dual-band vs narrowband filters: which suits your camera and targets?

29 October 2026 · 2 min read

Narrowband filters are the reason people image nebulae from city centres and under a full moon. They also make galaxies look worse. Here's how to choose.

What they pass

Emission nebulae glow at a few specific wavelengths:

A narrowband filter passes one line, usually 3–7 nm wide. A dual-band filter passes two lines in one piece of glass, most often Hα and OIII. Light pollution and moonlight are spread across the whole spectrum, so cutting everything else away boosts the contrast of the nebula enormously.

Colour camera or mono?

A colour (one-shot colour) camera has a Bayer matrix: in every 2 × 2 block of pixels, one is red, two are green and one is blue. A single Hα filter only reaches the red pixels, a quarter of the sensor. A dual-band filter is the better fit: Hα lands on the red pixels and OIII on the green and blue, so one exposure captures both.

A mono camera uses every pixel for whatever the filter passes, so separate Hα, OIII and SII filters give the most signal and let you build "Hubble palette" (SHO) images.

Which targets?

• Great with narrowband: emission nebulae (North America, Heart, Rosette, Lagoon), supernova remnants (Veil) and planetary nebulae (Dumbbell, Ring).
• Bad with narrowband: galaxies, reflection nebulae and star clusters. They shine in ordinary starlight, which these filters block. Shoot them with no filter, a UV/IR cut, or a broadband light-pollution filter.

Set your filters in the builder and choose My rig on the Tonight tab: targets that suit your filters rise to the top, and hovering a label explains why.

Narrower isn't always better

• 3 nm filters give the most contrast, but in a fast light cone (around f/3.5 or faster) the passband shifts and signal drops. Use filters made for fast optics, such as Optolong's "F2" versions.
• Dual-band on a colour camera still captures some broadband light, so star colours survive better than with pure narrowband.