Dual-band filters on smart telescopes: when they help and when they hurt
Emission nebulae glow in a few specific colours, chiefly hydrogen-alpha at 656 nm and doubly-ionised oxygen at 500 nm. Skyglow is spread across the whole spectrum. A dual-band filter passes narrow windows around those two lines and blocks the rest, so the nebula comes through nearly intact while most of the sky does not.
What the numbers say
The model treats the filter as passing 15% of the skyglow, 70% of an emission-line object's light and 15% of a broadband object's. With a Seestar S50:
| Target | Bortle 6, no filter | Bortle 6, dual-band | Bortle 8, no filter | Bortle 8, dual-band |
|---|---|---|---|---|
| Western Veil NebulaSupernova remnant | 2 h 28 min | 1 h 33 min | 6 h 59 min | 2 h 56 min |
| North America NebulaEmission nebula | 2 h 15 min | 1 h 25 min | 6 h 22 min | 2 h 41 min |
| Orion NebulaEmission nebula | 17 min | 12 min | 44 min | 21 min |
| Andromeda GalaxyGalaxy | 1 h 27 min | 17 h | 4 h 04 min | 35 h |
| Hercules ClusterGlobular cluster | 7 min | 1 h 19 min | 18 min | 2 h 29 min |
The Veil and the North America Nebula are pure emission objects and the filter cuts their time several-fold from town. The Orion Nebula is bright enough to be quick either way; the filter still helps its contrast. Andromeda and the Hercules cluster shine across the spectrum, so the filter blocks most of their light and keeps only a thin slice of it: a straight loss.
Which targets
- Yes: emission nebulae (North America, Pelican, Heart, Soul, Rosette, Lagoon, Eagle), supernova remnants (Veil, Crab), planetary nebulae (Ring, Dumbbell, Helix). The site's target pages recommend the filter for these automatically.
- No: galaxies, globular and open clusters, reflection nebulae (Pleiades, Witch Head) and dark nebulae. Everything that shines by starlight.
- Either: the Orion Nebula and other bright mixed regions. Filtered frames tame the core and lift the faint outer loops; unfiltered frames keep natural star colour. Many people shoot both and blend.
Which scopes have one
| Telescope | Dual-band | Filter details |
|---|---|---|
| ZWO Seestar S50 | Built in (LP filter) | Built-in dual-band light-pollution filter, switchable in the app. |
| ZWO Seestar S50 Pro | Built in (LP filter) | Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters on the telephoto camera. |
| ZWO Seestar S30 | Built in (LP filter) | Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters. |
| ZWO Seestar S30 Pro | Built in (LP filter) | Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters on the telephoto camera. |
| DwarfLab DWARF 3 | Built in (Dual-band) | Three built-in filters: VIS, Astro (430 to 690 nm) and dual-band Hα 656 nm / OIII 501 nm. |
| DwarfLab DWARF mini | Built in (Dual-band) | Built-in Astro (430 to 690 nm), dark-frame and dual-band Hα / OIII filters; no plain VIS filter on the tele camera. |
| DwarfLab Draco | Built in (Hα + OIII) | Built-in Hα + OIII dual-narrowband filter, 13 nm per line; the SHO Edition adds an SII option. |
| DwarfLab DWARF II | None | Two built-in filters, VIS and Astro (IR-pass); no dual-band. The optional UHC filter is no longer sold. |
| Vaonis Vespera II | Clip-on option (Dual Band filter) | Optional clip-on Vaonis filters: dual-band, light-pollution (CLS) and solar; the app detects which is fitted. |
| Vaonis Vespera Pro | Clip-on option (Dual Band filter) | Optional clip-on Vaonis filters: dual-band, light-pollution (CLS) and solar. |
| Vaonis Vespera 3 | Clip-on option (Dual Band filter) | Optional clip-on Vaonis filters: dual-band, light-pollution (CLS) and solar. The new optical baffle is not a substitute for a CLS filter. |
| Vaonis Vespera Pro 2 | Clip-on option (Dual Band filter) | Optional clip-on Vaonis filters: dual-band, light-pollution (CLS) and solar. |
| Unistellar eQuinox 2 | None | No built-in filter. Unistellar sells only a solar filter and says a third-party filter is at your own risk and voids the warranty. |
| Unistellar Odyssey | None | No filter option; Unistellar says third-party filters cannot be fitted to the Odyssey. |
| Celestron Origin | Drawer option (Nebula Filter) | Filter drawer takes 1.25-inch or 2-inch filters, including dual-band; Celestron sells an optional Nebula Filter. |
| Celestron Origin Mark II | Drawer option (Nebula Filter) | Filter drawer takes 1.25-inch or 2-inch filters, including dual-band; Celestron sells an optional Nebula Filter. |
Using it well
- Pair it with longer subs. Once the filter has removed the skyglow, read noise becomes a real share of each frame's noise and 30 to 60 s subs pay even from town. See how long subs should be.
- It beats the Moon. Moonlight is broadband, so an emission nebula behind a dual-band filter is the natural target for the bright half of the month.
- Expect a red and teal palette. Two bands means two colours. Star colours suffer; the nebula does not.
- Do not leave it on by accident. The most common reason a galaxy comes out dim and noisy on a Seestar is a filter left switched in from the previous target.
Every maker names its filters differently (ZWO's app calls this one the LP filter, DwarfLab's Astro position is the no-filter state). The filters guide has the full list and what the other positions do.
More guides
- Which smart telescope should you buy?
- How long should my subs be?
- Integration time explained
- The Bortle scale for smart telescope owners
- EQ mode and field rotation on smart telescopes
- Filters explained: VIS, Astro, UV/IR-cut, dual-band, CLS and solar
Updated 2026-08-27. Corrections: about page.