Astronomik CLS 1.25″ Broadband Light Pollution Filter

Astronomik CLS 1.25″ Broadband Light Pollution Filter

The Astronomik CLS (City Light Suppression) is an affordable broadband filter designed for visual observation and both monochrome and colour astrophotography of nebulae, galaxies and star clusters. It works with telescopes of any aperture and any focal ratio between f/3 and f/15, making it one of the most universally applicable light pollution filters available. The CLS filter selectively blocks the emission lines of mercury and sodium vapour street lamps - the most common sources of artificial light pollution - while transmitting the majority of the visible spectrum and the critical H-alpha band at 656 nm. The result is noticeably improved contrast on extended deep-sky objects, even from suburban or urban observing sites.

How the CLS Filter Works

Light pollution from urban environments is dominated by the spectral emission lines of mercury and high-pressure sodium vapour lamps used in street lighting. The Astronomik CLS filter is precision-engineered to block exactly these wavelengths while leaving the rest of the visible spectrum - including the emission lines most important for nebula observation and photography - largely unaffected. The dark-adapted human eye perceives most of the transmitted light naturally, so the visual experience remains bright and natural while the unwanted background glow is significantly reduced.

Transmission is optimised for the most important astronomical emission lines: 92 % at H-beta (486 nm), 92 % at both OIII lines (496 nm and 501 nm), and 97 % at H-alpha (656 nm). The filter passes wavelengths in the range 450-540 nm and above 650 nm, covering the full range of emission nebula lines visible to the eye and detectable by camera sensors.

Recommended Applications

The Astronomik CLS filter performs differently depending on the type of sky and the imaging equipment used. Here is a practical overview:

Visual observation from dark skies - Good. Reduces the glow from mercury street lamps and improves contrast on emission nebulae and galaxies.

Visual observation from urban or suburban skies - Good, though in this case the Astronomik UHC-E or UHC filters offer even stronger suppression of broadband light pollution and may be a better choice.

Classical film photography - Very good. Near-perfect colour balance is maintained across the transmitted spectrum.

CCD cameras - Good, provided an additional infrared blocking filter is used alongside the CLS to prevent IR contamination of the image.

Unmodified DSLR cameras - Good. White balance may be slightly less precise than with a modified camera, but image contrast is excellent.

Astro-modified DSLR cameras - Very good. With the camera's internal IR-cut filter removed, colour balance is nearly perfect.

MC-modified DSLR cameras - Good, when used together with an additional IR blocking filter.

Webcam / video - planets - Not recommended. The CLS filter is not designed for high-contrast planetary work.

Webcam / video - deep sky - Very good, particularly where light pollution is a significant problem.

Optical Quality and Construction

Astronomik filters are manufactured to the highest optical standards. The CLS filter is parfocal with all other Astronomik filters, so no refocusing is needed when switching between filters during an imaging session. The optical glass is just 1 mm thick, which eliminates any risk of diffraction-induced degradation of your telescope's optical performance - a common problem with thicker filters from other manufacturers. Light loss and chromatic aberration, which affect many competing products at faster focal ratios, only become relevant with Astronomik filters at f/2 or faster.

The filter substrate is fully resistant to high humidity, scratching and ageing, ensuring long-term reliability even in demanding field conditions. Every Astronomik filter is supplied in a high-quality, robust protective case that keeps it safe during storage and transport.

Technical Specifications