Astronomik CLS 2″ filter
Do you live in or near a city where light pollution ruins your observing experience? The Astronomik CLS 2″ filter is designed precisely for this situation. This broadband filter suppresses the unwanted emissions from sodium and mercury vapour lamps while passing the light from astronomically interesting objects - nebulae, galaxies and star clusters. The result is noticeably higher contrast and a more rewarding session at the eyepiece or behind the camera.
How the CLS filter works
CLS stands for City Light Suppression. The filter operates on the principle of selective transmission - it blocks wavelengths associated with artificial lighting, in particular:
- Sodium vapour lamps around 589 nm
- Mercury vapour lamps at 546 nm and 577 nm
At the same time it passes the natural emission lines from deep space, including:
- H-alpha at 656 nm
- H-beta at 486 nm
- OIII doublet at 496 nm and 501 nm
This makes the filter genuinely versatile - it performs well on emission nebulae, galaxies and open clusters alike, without stripping away the natural colours of the stars in the field.
Optical quality and mechanical design
The filter uses multi-layer vacuum-deposited coatings on optical glass. This ensures minimal reflections and high transmission in the desired passbands, with consistent performance maintained throughout the product's lifetime. The 2″ aluminium threaded ring provides a firm, precise fit in any eyepiece or camera adapter with a standard 2″ thread, with no play and no risk of scratching the eyepiece barrel. The filter is also compatible with step-down adapters for use in 1.25″ accessories.
Visual observing
At the eyepiece the difference is immediately noticeable. The sky background appears darker, objects are more sharply defined, and faint detail in nebulae becomes easier to hold. If you are observing from a suburban location with a Bortle class 5 sky or worse, a light pollution suppression filter of this type is no longer a luxury - it is a practical necessity.
Astrophotography
For imaging, the CLS filter significantly shortens the exposure time needed to achieve a usable histogram without an overexposed background. It is compatible with:
- DSLR cameras
- Mirrorless cameras
- Dedicated astronomical CCD and CMOS cameras (colour and monochrome)
When used with colour cameras, the filter preserves the natural colour rendering of stars far better than narrowband filters do. Final images look natural and aesthetically pleasing without sacrificing scientific value.
Key specifications
| Parameter |
Value |
| Filter type |
Broadband light pollution suppression (CLS) |
| Filter size |
2″ |
| Blocked wavelengths |
Sodium ~589 nm, Mercury ~546 nm and ~577 nm |
| Passed wavelengths |
H-alpha 656 nm, H-beta 486 nm, OIII 496/501 nm |
| Coating |
Multi-layer vacuum-deposited |
| Frame material |
Aluminium, threaded ring |
| Compatible threads |
2″ standard; adaptable to 1.25″ |
| Use case |
Visual observing and astrophotography |
| Suitable cameras |
DSLR, mirrorless, CCD, CMOS |
| Recommended sky conditions |
Bortle class 5 and above (suburban and urban) |
| Manufacturer |
Astronomik (Germany) |
About Astronomik
Astronomik is a German manufacturer of astronomical filters with a long-standing reputation for optical precision. Every filter undergoes strict quality control, and the multi-layer coatings are applied using a vacuum process that guarantees consistent parameters over the entire service life of the product. Astronomik filters are trusted by visual observers, beginner astrophotographers and experienced astronomers who cannot afford compromises in optical quality. The CLS filter represents an accessible entry point into the world of serious light pollution filtration - and once you have experienced the difference, returning to unfiltered observing becomes difficult to justify.
