Explore Scientific CLS 2″ - broadband light pollution filter
Do you live in a city or its surroundings where the night sky glows with the orange haze of street lighting? The Explore Scientific CLS (City Light Suppression) filter is exactly what you need. This broadband filter is designed to suppress unwanted light pollution originating from high-pressure sodium and mercury vapour lamps - the typical source of the orange and yellow glow over urban areas. The result is a significantly higher contrast between the sky background and the objects you are observing: nebulae, galaxies and other deep-sky targets.
How the filter works
The filter operates on the principle of selective light transmission. It blocks specific emission lines of sodium lamps (NaI, 589 nm) and mercury lamps (HgI, 405 nm and 436 nm), while transmitting the wavelengths that matter most for astronomical observation - primarily the H-alpha region (656 nm), H-beta (486 nm) and OIII (496 nm and 501 nm).
This makes the filter effective for both visual observation and astrophotography, where it can significantly reduce the exposure time needed to achieve a usable histogram without an overexposed background. The filter is intended primarily for observers and photographers working from locations with moderate to heavy light pollution, but it will also be appreciated by those who want to maximise contrast when photographing nebulae and galaxies even from darker sites.
Build quality and compatibility
The filter is mounted in a robust aluminium cell in the standard 2″ diameter, which fits the vast majority of focusers, filter wheels and camera adapters. The thread conforms to the standard 2″ filter thread. The machining of the cell is precise - the filter threads on easily and holds firmly in place even during longer exposures. The optical glass features high-quality anti-reflection coatings that minimise unwanted reflections and ghost images that could degrade a photographic frame or the visual impression.
What you can observe and photograph with the CLS filter
The Explore Scientific CLS filter is useful for visual observation of diffuse emission nebulae (Orion Nebula M42, Lagoon Nebula M8, and others), galaxies and star clusters against a light-polluted sky. In astrophotography it allows you to capture images with more natural colours than narrowband filters such as H-alpha or OIII, while still offering noticeable suppression of stray light. It is therefore an ideal choice for:
- Beginner astrophotographers who want quick results without the need for long exposure sequences
- Experienced observers looking for a versatile everyday filter from suburban locations
- Anyone wanting to reduce heavy gradient correction work in post-processing
Use with cameras and advanced setups
If you shoot with a DSLR or mirrorless camera with a Bayer matrix, the CLS filter will significantly reduce the need for aggressive gradient removal in post-processing. When used with planetary cameras or dedicated astro cameras in combination with narrowband filters, the CLS can also serve as a pre-filter for rough suppression of the sky background. It is therefore a versatile filter for astronomers observing from the city that will quickly earn a permanent place in your kit.
Key specifications
| Parameter |
Value |
| Filter diameter |
2″ |
| Filter type |
Broadband light pollution suppression (CLS) |
| Thread |
Standard 2″ filter thread |
| Cell material |
Aluminium |
| Transmitted wavelengths |
H-alpha 656 nm, H-beta 486 nm, OIII 496/501 nm |
| Blocked wavelengths |
NaI 589 nm, HgI 405 nm and 436 nm |
| Coating |
Multi-layer anti-reflection |
| Use |
Visual observation and astrophotography |
| Recommended for |
Urban and suburban observation sites |
