Dual / Multi-Band Filters (Color)

Choosing Bandwidth

• 3 nm: Maximum contrast and the smallest stars, best under heavy light pollution and bright Moon. Needs longer exposures. Optolong L-Ultimate, Antlia ALP-T 3 nm, SVBONY SV220 3 nm, Askar E1.
• 5 to 7 nm: The best balance for most users, with good contrast and reasonable exposures. Optolong L-Extreme, Antlia ALP-T 5 nm, SVBONY SV220 7 nm.
• 10 nm and wider, multi-band: Brighter and more forgiving, passes H-beta too in some designs. Gentler star colour. Optolong L-Enhance, L-Quad Enhance, L-Para.
• SII/OIII filters: A second filter such as Optolong L-Synergy or Askar C2, D2 and E2 adds sulphur data for bicolour or Hubble-palette-style images with a colour camera.

Fast Optics and Filter Choice

In fast systems below about f/4, light enters the filter at steeper angles and the passband shifts toward blue. Very narrow filters can then cut into the emission line. For RASA, HyperStar and fast Newtonians, choose a filter rated for fast optics, often called Highspeed.

What We Check With You

• Sky brightness and targets: Bortle 8 suburbs and bright emission nebulae call for 3 to 5 nm. Darker skies leave room for wider filters.
• Focal ratio: We match the filter design to the speed of your telescope.
• Size and mounting: 1.25-inch, 2-inch or a filter drawer, depending on your sensor and corrector.

Dual-Band Filters: Frequently Asked Questions

Do dual-band filters work on galaxies?

Not well. Galaxies emit across the whole spectrum, so a dual-band filter blocks most of their light. Use them for emission nebulae, and a broadband filter or dark skies for galaxies.

Can I use a dual-band filter with a mono camera?

Technically yes, but a mono camera cannot separate the two lines. Single-line narrowband filters are better for mono.

Why are my stars red or pink with a dual-band filter?

Stars are broadband, so only a little of their light passes. Processing tools that reduce or recolour stars help a lot.

Imaging nebulae from the city?

Tell us your camera, telescope focal ratio and sky. We will recommend the bandwidth that suits your setup.