Astronomik CLS 2" (M48)

The Astronomik CLS filter is designed for visual observation of nebulae, galaxies, and star clusters using telescopes. It completely blocks the emission lines from high- and low-pressure sodium (Na)- and mercury (Hg)-vapor lamps as well as all lines of natural airglow while allowing the majority of visible light and nebulae emissions to pass through. This enhances the contrast between deep-sky objects and the night sky, making it ideal for observation in light-polluted areas.

Due to its wide passband, the Astronomik CLS filter passes a substantial amount of light, minimally dimming stars. The filter is designed to block as much unwanted light as possible while still transmitting the maximum amount of light from astronomical objects.

Other uses

The Astronomik CLS Filter can also be used for photography from light-polluted areas.

Alternatives

The Astronomik UHC filter is particularly suitable for observing emission nebulae under light pollution and dark skies.
The Astronomik UHC-E filter is better suited for observing emission nebulae and comets under moderate light pollution.

Suitability of the Astronomik CLS filters

Visual observation (dark skies): Rather unsuitable
Visual observation (urban skies): Reasonable, particularly suitable for heavy light pollution

Technical Data

92% transmission at 486nm (H-beta)
92% transmission at 496nm (OIII)
92% transmission at 501nm (OIII)
97% transmission at 656nm (H-alpha)
Transmission range: 450 to 540nm and above 650nm
Glass thickness: 1mm (except Astronomik OWB Type 3 and XT filters)
Finely polished optical glass, free of any striae, residual stresses, and bubbles
Extremely scratch-resistant and durable coating
Completely resistant against high humidity and aging effects
Astronomik MFR coating (Astronomik Deep-Sky RGB, UV-IR Block L-1, L-2, L-3, and Narrowband-Emissionline filters)
Parfocal with other Astronomik filters
Diffraction-limited
Optimized spectral windows
Delivered in a high-quality, long-lasting filter box

Due to the high-precision machining of the carrier substrate, all Astronomik filters are parfocal with wedge errors far below all common tolerances. Astronomik filters are diffraction-limited, ensuring that the optical performance (resolution and contrast) of your telescope is not compromised. Issues concerning focal distances and the exact alignment of images from each color channel are therefore effectively eliminated.

All Astronomik filters are ideal for modern cameras, including both CMOS- and CCD-based models, ensuring you capture the best possible images with your equipment. Sharpness, contrast, and the absence of any reflections and halos are guaranteed. Even with bright stars in the field of view, nebulae and galaxies will be captured with the finest structures. The Astronomik MFR coating is designed for optimal performance across a wide range of focal ratios of the optics used.

Every Astronomik filter undergoes a rigorous quality control process before delivery. Additionally, each Astronomik Narrowband-Emissionline filter is measured using a high-end PerkinElmer Spectrophotometer to ensure it meets the specifications published on our website. On the back of each Narrowband-Emissionline filter box, you will find a label detailing the precise spectral data for that specific filter, including the maximum transmission and the recommended range of focal ratios.

This Product was added to our catalogue on 15/10/2025.

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