Explore Scientific 1.25″ CLS filter - see deeper from the city
If you observe from an urban or suburban location, you know how frustrating it can be to search for faint nebulae against a washed-out orange glow. The Explore Scientific CLS (City Light Suppression) filter is designed precisely for these conditions - helping you recover contrast and detail without always having to travel to a remote dark-sky site.
How the CLS filter works
The CLS filter operates by selectively blocking the wavelengths of light most commonly produced by artificial lighting - in particular high-pressure sodium and mercury vapor lamps. These are the dominant sources of light pollution above most cities, and their emissions are concentrated into well-defined spectral lines. By suppressing these lines while transmitting the wavelengths where the most interesting astronomical objects radiate, the filter delivers noticeably higher contrast between the target object and the sky background.
As a broadband filter - as opposed to narrowband filters such as H-alpha or OIII - the CLS passes a relatively wide portion of the visible spectrum. This means the image remains sufficiently bright and comfortable for the eye, making it an excellent choice for observers who want to use a single filter for both visual astronomy and astrophotography under moderate light pollution.
What you can observe with the CLS filter
The CLS filter works particularly well on emission nebulae and planetary nebulae. Some objects where the improvement is especially noticeable include:
- M97 - the Owl Nebula in Ursa Major
- NGC 6992 - the Eastern Veil Nebula in Cygnus
- M27 - the Dumbbell Nebula in Vulpecula
Even from a suburban backyard, it becomes possible to perceive the structure and extent of these objects in a way that would simply not be achievable without the filter under the same conditions. For astrophotography, the CLS filter also extends effective exposure times by reducing the uneven brightness gradient caused by light-polluted skies.
Build quality and compatibility
Explore Scientific CLS filters are individually certified, with each unit accompanied by a certificate guaranteeing its optical quality. The filter housing is machined from aluminum and uses the standard 1.25″ thread, allowing quick and secure attachment to any compatible eyepiece or camera adapter. The optical surfaces are precision-ground and feature a multi-layer coating that minimizes reflections and light loss outside the blocked wavelengths.
Key specifications
| Parameter |
Value |
| Filter type |
Broadband - CLS (City Light Suppression) |
| Thread size |
1.25″ |
| Housing material |
Aluminum |
| Blocked wavelengths |
High-pressure sodium and mercury vapor emission lines |
| Suitable for |
Visual observation and astrophotography |
| Certification |
Individual optical quality certificate included |
Is the Explore Scientific CLS filter right for you?
If you are looking for a practical and reliable way to improve your observing sessions from less-than-ideal locations, the Explore Scientific 1.25″ CLS broadband filter offers a strong balance of price, quality and real-world performance - whether you are observing visually or shooting with a camera attached to your telescope. It is an investment that makes the night sky accessible even where light pollution would otherwise dominate.
