CLS vs. Dual-Band filters: how to choose and use them effectively

At Vaonis, we often get questions from users who want to optimize their observations using filters — particularly in areas affected by light pollution. No matter your level of experience with astrophotography, there’s value in knowing how to get the most from your equipment, of how and when to use CLS and Dual-Band filters can make all the difference.

In this post, we break down their differences, their ideal use cases, and tips to get the most out of your observing sessions.

CLS Filter (City Light Suppression)

What does it do?

The CLS filter is a broadband filter designed to reduce the overall glow from light pollution, particularly helpful in urban or suburban environments.

When to use it

• Observing from light-polluted locations (Bortle scale 6+)

• When trying to reduce the sky glow caused by urban lighting

• For general deep-sky objects like galaxies, clusters, and some nebulae
  
  

When not to use it

Since the CLS filter dims part of the visible spectrum, it can reduce the brightness of your target objects — especially galaxies. Avoid using it if your sky is relatively dark (Bortle 5 or lower).

Dual-Band Filter

What does it do?

This is a narrowband filter that only allows light from two specific emission lines through:

• OIII (Oxygen III) – 500.7 nm
  

• Hα (Hydrogen Alpha) – 656.3 nm
  

It blocks most of the rest of the spectrum, making emission nebulae stand out dramatically.

When to use it

• When targeting emission nebulae like the North America Nebula, the Rosette Nebula, or the Heart Nebula

• From locations with significant light pollution — it performs even better than the CLS filter for this purpose
  
  

When not to use it

• Not suitable for galaxies, star clusters, or reflection nebulae

• In mixed nebulae (e.g., Trifid), you’ll lose part of the signal, especially in reflection zones

Tips for Using Filters

Filter Installation

• You can install the filter before or after initialization — it won’t impact focus

• Avoid removing or changing the filter during a session, as this disrupts image stacking.
  
  

Astrometric Calibration

Dual-Band filters can sometimes make plate solving difficult in star-poor regions. If pointing fails, try initializing without the filter.

Final Thoughts: which filter should you use?

Each filter has a specific role, and using the right one for the right target can significantly enhance your results — especially in light-polluted skies.

Have more questions?

Share your favorite emission nebula to observe with the Dual-Band filter in the comments, or reach out to our community — we’re always happy to help!