Omegon Clear Sky Filter 1.25" - CLS Filter for Visual Observation and Astrophotography
The night sky is never truly dark. Even far from the nearest town, the sky has a natural glow - a faint but persistent luminescence caused by processes in the upper atmosphere. Add the orange haze of city and suburban lighting, and the background sky brightens to a level that washes out contrast and hides all but the brightest deep-sky objects. The Omegon Clear Sky is a CLS (City Light Suppression) filter designed to tackle both of these problems simultaneously - cutting through artificial light pollution and blocking the natural airglow that affects observers even at genuinely remote sites.
What Is Airglow and Why Does It Matter?
Most amateur astronomers are familiar with light pollution - the artificial glow from street lamps, industrial facilities, and illuminated buildings. Fewer are aware of airglow, a natural phenomenon that brightens the night sky regardless of how far you travel from civilisation. Airglow is caused by ultraviolet and X-ray radiation from the Sun exciting molecules in the upper layers of the Earth's atmosphere. These excited molecules release energy in the form of faint light, predominantly in specific wavelengths including the yellow-green emission of oxygen around 557.7 nm and the strong hydroxyl (OH) emission bands in the near-infrared.
The result is a diffuse background glow that reduces the contrast between faint deep-sky objects and the sky background, even on moonless nights at dark sky sites. The Omegon Clear Sky filter blocks these specific wavelengths while transmitting the light from astronomical objects, effectively darkening the sky background and making faint objects easier to detect and observe.
What the Clear Sky Filter Blocks - and What It Lets Through
The Omegon Clear Sky filter is a broadband CLS design, meaning it transmits a wide range of visible wavelengths while selectively blocking the emissions responsible for sky glow. The filter suppresses the spectral lines of mercury-vapour and sodium-vapour street lamps - the dominant sources of artificial light pollution in most locations - as well as the airglow wavelengths from the upper atmosphere. At the same time, it transmits the important emission lines from astronomical objects: hydrogen-alpha (H-alpha), hydrogen-beta (H-beta), and doubly ionised oxygen (OIII), which are the primary wavelengths emitted by emission nebulae, supernova remnants, and planetary nebulae.
Because the Clear Sky filter uses a broad passband rather than an extremely narrow one, it also transmits the reflected light from star clusters, galaxies, and comets without significantly dimming these objects. Star colours remain largely natural, making the filter pleasant to use visually without the unnatural appearance that very narrow filters can produce.
Suitable Objects and Observing Conditions
The Omegon Clear Sky filter delivers visible improvement across a wide range of deep-sky objects. Emission nebulae - including large objects like the Orion Nebula (M42), the Lagoon Nebula (M8), and the North America Nebula - show more clearly defined structure and improved separation from the sky background. Planetary nebulae such as the Ring Nebula (M57) or the Dumbbell Nebula (M27) benefit from higher contrast against the background sky. Supernova remnants, including the Veil Nebula complex, are among the most dramatic examples of what a CLS filter can reveal - objects that are barely detectable without filtration can become clearly visible with the Clear Sky in place.
Star clusters and galaxies also benefit from the darker sky background, even though they do not emit light in the specific wavelengths the filter preferentially passes. Observing and photographing the Milky Way from a light-polluted site is noticeably more rewarding with the Clear Sky filter in use.
Ideal for Smaller Telescopes
The Omegon Clear Sky filter is particularly well matched to telescopes with apertures up to approximately 150 mm. This is an important consideration when choosing between filter types. A very narrow narrowband filter - such as an OIII or H-alpha filter - restricts the light so severely that smaller telescopes struggle to produce a bright enough image for comfortable visual observation. The broader transmission of the Clear Sky filter preserves image brightness while still providing meaningful contrast improvement, making it the right choice for 60 mm to 150 mm refractors, Maksutov-Cassegrain telescopes, and smaller Newtonians. For larger instruments, the filter is still useful but the additional aperture also makes narrowband options more viable.
Astrophotography With the Omegon Clear Sky
The filter is specified as suitable for astrophotography, and it performs well with both DSLR cameras and dedicated astronomy cameras in one-shot colour mode. Because of its broad passband, it does not introduce the strong colour casts that narrowband filters create, and the resulting images can be processed with standard techniques without specialist narrowband workflows. For photographers shooting emission nebulae, the Milky Way, or wide-field sky regions from light-polluted locations, the Clear Sky filter provides a practical and affordable way to improve image quality without the complexity of narrowband imaging.
Construction and Compatibility
The Omegon Clear Sky filter is mounted in a standard 1.25" aluminium cell with a threaded ring, screwing directly into any 1.25" eyepiece, star diagonal, or filter holder. The optical glass is produced to flat and parallel tolerances to avoid any image distortion or focus shift. Both surfaces carry multi-layer anti-reflection coatings to minimise light loss at the glass-air interfaces and suppress internal reflections that would otherwise reduce contrast. The filter is part of the Omegon Advanced series, reflecting a higher standard of optical and mechanical quality than entry-level products.
Technical Specifications
- Filter type: CLS (City Light Suppression) - broadband
- Barrel size: 1.25"
- Frame material: aluminium
- Optical coatings: multi-layer anti-reflection
- Series: Omegon Advanced
- Visual use: yes
- Astrophotography: yes
- Anti light-pollution: yes
- Suitable for emission nebulae: yes
- Suitable for planetary nebulae: yes
- Suitable for supernova remnants: yes
- Suitable for hydrogen nebulae (H-alpha): yes
- Recommended aperture: up to approx. 150 mm
