How To Combine Narrowband Filters For Richer Nebula Images

If you’re using a colour astronomy camera like I do under less-than-perfect skies, you already know the challenge: light pollution, sky glow, and broad-spectrum noise all conspire to mask the faint emission lines of nebulae. Narrowband filters help by blocking unwanted light and passing only the wavelengths your target emits. But even within that category, there’s room for strategy – combining different narrowband filters to get both flexibility and depth.

One promising strategy is to use a filter like the new Optolong L‑Synergy – a dual narrowband pass for OIII + SII – in tandem with a stricter dual filter such as Optolong L‑Ultimate (3 nm, Ha + OIII). In effect, this pairing gives you a broader set of emission data while still taming sky glow and improving contrast.

Let’s explore when and how to use this approach, why it works, and where it’s limited.

Understanding What Each Filter Does

The L‑Synergy filter is designed to pass two emission lines: OIII (at about 500.7 nm) and SII (about 672.4 nm), while rejecting much of the background light around them. (Optolong describes it as a 7 nm dual narrowband filter).

In contrast, the L‑Ultimate filter passes Ha (656.3 nm) and OIII (500.7 nm), but with a much tighter band‑width (3 nm), making it more selective and yielding even lower background noise.

So one filter favours SII + OIII, while the other favours Ha + OIII, both in a narrow band.

Why pair them? Because together they can give you OIII, Ha, and SII information – the essential trio for many nebulae – without needing a full 3‑filter setup (Ha / OIII / SII). Paired use can let you reconstruct multispectral data, enriching structure and contrast.

Optolong even sells a combo set (L2 Dual‑Combo) that includes both an L‑Synergy and an L‑eXtreme (Ha + OIII), so the concept is officially supported.

What You Gain by Pairing Filters

Here are the benefits you may see in real use:

• More emission coverage: You get separate SII and Ha channels, as well as OIII, letting you map nebula chemistry more richly than with a single dual filter.
• Better contrast in polluted skies: The L‑Ultimate’s tighter pass allows deeper suppression of sky glow, so your Ha signal stands out more.
• Efficiency: You don’t have to switch filters for every emission line; each filter gives two lines.
• Creative flexibility: With the combined data you can create more complete colour mappings or experiment with different palettes.
• Reduced halo and artefacts: Each filter can help mitigate side effects caused by bright stars or residual gradient, especially when used in balance.

How To Use Them Together – Practical Workflow Suggestions

Here’s how an astrophotographer might integrate both:

1. Session planning: Choose a nebula rich in all three lines (Ha, OIII, SII) – for example, the Heart & Soul Nebulae or parts of the North America Nebula. Plan to image with both filters during the same night (or across nights with similar conditions).
2. Acquisition strategy:
   • Use L‑Synergy for frames capturing OIII + SII, giving you a basis for those lines.
   • Use L‑Ultimate for Ha + OIII, focusing on capturing strong Ha signal with lower background.
   • Use similar exposure times, guiding, temperature regimes, and calibration frames (darks, flats, bias) for both sets.
   • Aim to match framing and orientation carefully so the data aligns well in post.
3. Stacking and combining:
   • Stack your L‑Synergy frames into a master SII + OIII image.
   • Stack L‑Ultimate frames into a Ha + OIII master.
   • Register both stacks together (align stars) so they overlay precisely.
   • Use processing software to separate and map Ha, OIII, and SII channels – for example, you might assign Ha to red, OIII to green or blue, and SII to another channel.
   • Blend the two master images judiciously – using masking, gradients, or weighting to balance contrast and minimise artefacts.
4. Fine tuning:
   • Use star masks to limit colour bleed.
   • Adjust relative scaling so no one channel dominates.
   • Watch for vignetting, banding, or halo in either set and correct with flats and gradient tools.

When This Pairing Works Best – And When It Doesn’t

Ideal when:

• You’re imaging from suburban or light‑polluted skies.
• Your target emits in Ha, OIII, and SII lines.
• You have a colour camera and want more colour information than a single dual narrowband can deliver.
• You can dedicate time for capturing two datasets and aligning them.

Less ideal when:

• You’re under extremely dark skies – a simpler narrowband or broadband setup may suffice.
• Your target is not rich in all three lines – e.g. pure reflection nebulae or galaxies.
• You don’t have the stability or alignment precision needed to combine the two datasets well.
• Your optics are very fast (< f/3), where filter bandshift becomes a problem.

Things to Watch Out For

• Mis‑alignment or rotation between datasets makes combining tricky.
• Differences in transparency, sky brightness, or guiding quality between the two sets can create mismatched noise or artefacts.
• Overprocessing can introduce false colour or imbalance.
• Bright star halos might be more visible in one filter than the other – good masking and stacking discipline help.

Final Thoughts: Expanding What You Can Capture

Using a filter like L‑Synergy together with a 3 nm dual filter (L‑Ultimate) gives you a powerful middle ground. You get more spectral depth – Ha, OIII, and SII – with fewer filter swaps than a full three‑filter narrowband setup.

It’s not easy. It adds workload and demands rigour in acquisition and processing. But for imagers working under challenging skies who want richer nebular data without overhauling their system, it offers a compelling path forward.

If you try this pairing, you’ll likely see parts of your nebula that were previously invisible – subtle sulphur filaments, contrasting oxygen regions, and more structure in deep gas clouds.

The sky gives more than our eyes see. With strategy, patience, and the right combination, you can discover more too.