Astronomik CLS Filter

Astronomik CLS Filter 1,25" (M28.5)

Astronomik CLS Filter 1,25" (M28.5)
Astronomik CLS Filter 2" (E48)
Astronomik CLS Filter 31mm, unmounted
Astronomik CLS Filter 36mm, unmounted
Astronomik CLS Filter 38mm, unmounted
Astronomik CLS Filter 50mm, unmounted
Astronomik CLS EOS Clip-Filter
Astronomik CLS Filter SC Rear Cell (2" / 24TPI)
Astronomik CLS Filter T-Mount (M42x0,75)
Astronomik CLS Filter Transmission Curve

Click the image for full resolution

The Astronomik CLS is a budget filter for visual observation, Black & White photography and CCD photographs of nebulae, galaxies and star clusters with astronomical instruments of any size.

The Astronomik CLS 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.

20s exposures with JPEG data directly out of the camera without any further processing. When the  mouse is moved over the image the image with the Astronomik CLS is shown. (Canon EOS 6D astro-modified, ISO 6400, Canon EF24-105mm f/4L IS II USM @ 24mm/4)

If you wonder what to expect from the CLS, you might want to watch this video from Kamil Pekala

Main use

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.

more information about the visual Astronomik filters

more information about the photographical Astronomik filters


How to read the chart
  • The horizontal axis is the Wavelength in Nanometers (nm). 400nm is deep blue, at 520nm the human eye senses green and at 600nm red. At 656nm is the famous "H-Alpha" emission line of hydrogen.
  • The transmission in % is plotted on the vertical axis.
  • The red line shows the transmission of the filter.
  • Visual filters: The grey line in the background shows the relative sensitivity of the human eye at night. The maximum is at ~510nm and drops to longer and shorter wavelengths. You can easily see, that you can´t see anything of the H-alpha line at night (even if you can during daylight!) The sensitivity at 656nm is 0% at night!
  • Photographic filters: The grey line in the background shows the sensitivity of a typical CCD sensor.
  • The most important artifical emission lines are shown in orange. The artifical light pollution is dominated by see mercury (Hg) and sodium (Na), which are used in nearly all streetlights.
  • The most important emission lines from nebulas are shown in green. The most important lines are from ionized Hydrogen (H-alpha and H-beta) and double ionized oyxgen (OIII).

The major emission lines of artifical light pollution:
| Hg 435,8nm | Hg 546,1nm | Hg 577,0nm | Hg 578,1nm |
| Na 589,0nm | Na 589,6nm | Na 615,4nm | Na 616,1nm |

The major emission lines of nebulas:
H-β 486,1nm | OIII 495,9nm | OIII 500,7nm | H-α 656,3nm


  • Visual observation (dark skies): Good, to reduce light pollution by mercury-vapour lamps (streetlight)
  • Visual observation (urban skies): Good, an UHC-E or UHC filter is more suitable
  • Film photography: Very good, colour balance is near perfect
  • CCD photography: Good, when used with an additional IR-block-filter
  • DSLR photography (original): Good, colour balance shifted but contrast enhanced
  • DSLR photography (astro modified): Very good, colour balance is near perfect
  • DSLR photography (MC modified): Good, when used with an additional IR-block-filter
  • Webcam / Video (Planets): Unsuitable
  • Webcam / Video (Deep Sky): Very good, if light pollution is a big problem

Technical Data

  • 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
  • Completely 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
  • Since 2008 we do ship filters with a completely new design. Any kind of halo or strange reflection is a matter of past

Gallery with images made using our Astronomik Filter:

Please visit our gallery for more technical information!

Mario Spénard: "I want to thank you for your magnificent CLS-CCD. I am using it with a standard unmodified Nikon D610 in a very light polluted area with some really nice result."

The filter is available in the following sizes:
(This filter is also available in various other sizes for astrophotography. They are listed on the page about filters against light pollution for astrophotography )

Filter displayed products below by their features

CLS Filter

  1. Astronomik CLS M72

    Astronomik CLS M72

    excl. VAT (Non-EU): €326.89 incl. VAT (EU): €389.00

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