Narrowband Filters

Emission Lines, Bandwidth and Focal Ratio

Narrowband filters isolate the specific wavelengths that ionized gas emits. Narrower is not always better, because the passband shifts in a fast light cone and the exposure requirement rises.

• Hydrogen-alpha at 656.3 nm: The strongest and most widely used emission line, showing nebulosity in extraordinary detail and working well even under a bright Moon.
• Oxygen III at 500.7 nm: Reveals planetary nebulae, supernova remnants and the outer shells of emission nebulae; typically the weakest signal of the three and needs the most integration.
• Sulphur II at 672.4 nm: The third channel of the Hubble palette, adding structural differentiation in bright nebulae.
• 3 to 5 nm bandwidth: Maximum contrast under light pollution, best suited to f/5 and slower systems and to accurate, temperature-compensated focus.
• 6 to 12 nm bandwidth: More forgiving of fast optics and shorter exposures, and a better match for one-shot colour cameras and systems faster than f/4.
• Dual and multi-band filters: Pass two or more lines simultaneously for one-shot colour cameras, giving narrowband-like contrast without a filter wheel.

Why Choose David Astro for Narrowband Filters

A filter that is wrong for your focal ratio will underperform in a way that looks like a quality problem but is a specification problem.

• Focal ratio matched: Below roughly f/4 the incident cone shifts a narrow passband off the emission line. We recommend bandwidths or fast-optic designs that account for this.
• Sizing to sensor and wheel geometry: We calculate the filter size needed to avoid vignetting given your sensor diagonal and filter-to-sensor distance.
• Focus offsets explained: Narrowband channels often need per-filter focus offsets, and we help set them up so you are not refocusing manually every filter change.
• Authorized dealer: Genuine filters with verified coatings and full manufacturer warranty, in Canadian stock.

Narrowband Filters in Canada: Frequently Asked Questions

Can I use narrowband with a colour camera?

Yes, most effectively with a dual-band filter that passes Ha and OIII together, since a colour sensor's Bayer array can separate them. Single-line filters work but waste three quarters of the pixels.

Is a 3 nm filter always better than 7 nm?

Not in fast systems. Below about f/4 the steep light cone shifts the passband and a 3 nm filter can lose signal. In an f/6 system under city skies, 3 nm gives noticeably better contrast.

Can I image through a full Moon?

With Ha, largely yes. Moonlight is broadband scattered sunlight, and a narrow Ha filter rejects nearly all of it. OIII suffers more because its wavelength sits closer to the peak of scattered light.

How long should narrowband exposures be?

Longer than broadband, typically 300 to 600 seconds per sub, depending on focal ratio and camera read noise. Total integration for a good narrowband image is usually measured in tens of hours.

Imaging from a light-polluted site?

Tell us your telescope, camera and focal ratio and we will recommend the right bandwidths. Ships from Canada.