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Astronomik - CLS Light Pollution Filter - 2" Round Mounted


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Manufactured by: Astronomik


Astronomik - CLS Light Pollution Filter - 2" Round - Mounted
The Astronomik CLS Filter blocks the light of the spectral lines of mercury and sodium-vapor lamps and lets the largest part of the visible light and H-alpha emissions pass. All the important emission lines, as well as the spectral region that the very well dark adapted eye can see, can pass through the filter.

The filters are optimized for use with aperture ratios from 1:3 to 1:15. Transmission losses and chromatic distortions, which are problems with other filters, only appear with Astronomik filters when extremely bright aperture ratios of 1:2 and more are used. The CLS filter is suitable for use with telescopes of all aperture sizes. 

Superior Substrate:   Astronomik filters consist of a precision polished substrate on which the actual filter, in the form of very thin (dichroic) interference layers, is applied. Because of these high quality measures Astronomik filters will in no way reduce the performance of your optical system. And contrary to common experience with other manufacturers´ filters, with Astronomik filters stars remain fine needle points of light! 

Coating:   Astronomik filters consist of up to 130 layers deposited on top of the completely transparent carrier glass. These layers work as the actual filter and as an antireflective coating. 

Durability:   Astronomik filters are completely scratch resistant, and not sensitive to moisture, dew, or high humidity. With others’ filters, after a few years, you start to notice their filter is no longer up to its full capability. Not so with Astronomik filters which maintain their ‘brand new’ transmission specifications year after year.


92% transmission at 486nm (H-beta)
92% transmission at 496nm (OIII)
92% transmission at 501nm (OIII)
97% transmission at 656nm (H alpha)
Pass from 450 to 540nm and beyond 650nm


  • Parfocal with other Astronomik filters
  • Glass thickness: 1mm
  • Highly resistant against high humidity, scratches and aging effects
  • Diffraction limited, the filter will not reduce the optical performance of your telescope!
  • Astronomik filters are delivered in a high-quality, long lasting, filter box




Product Series

The Astronomik CLS is our inexpensive solution for the visual deep-sky observation, b&w photography and CCD observation. The Astronomik CLS filter is the right filter to gain the contrast of nearly all objects under light-polluted skies with all sizes of telescopes. Due to the design of the transmission curve the filter reduces the sky background and gas nebulae planetary nebulae as well as open clusters, globular clusters and galaxies are seen better. The Astronomik CLS blocks the emission of Hg and Na streetlights as well as all Skyglow-lines . All major emission lines from deep-sky objects are passed without suppression. As you can see from the chart below the most important part of the eye's night-sensitive region is passed with maximum transmission.

The filter are optimized for the use with focal ratio from 1:4 to 1:15. Loss of transmission and color displacements, that appear at other filters, emerge with the Astronomik filters only at extreme fast focal ratios of 1:2 and bigger. Compared to the 'Deep-Sky' filters sold by other manufacturers the Astronomik CLS shows you more stars and more details in the nebulae. Because of the high quality of the glass you will see stars as the small pinpoints you know from your scope. The Astronomik CLS can be used with all sizes of instruments. Even the use in front of binoculars is a great improvement. * High transmission of nearly 100% between 450nm and 530nm * transmission of ca 96% at H-alpha * total blocking of all unwanted light from light-pollution * the ideal all around filter for deep-sky observing, even with small scopes * completely resistant against high humidity and scratches * highest optical quality on the market

Additional Information

Name Astronomik - CLS Light Pollution Filter - 2" Round Mounted
Manufacturer Astronomik
Model CLS-2