Astronomik CLS Visual Deep Sky & Light Pollution Filter 1.25-in

Astronomik 1.25-Inch CLS Deep Sky & Light Pollution Filter for Visual Observations, B&W and CCD Photography
Mounted filter in 1.25" cell ( M28.5mm x 0.5mm )

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.

Astronomik has re-designed all of their photographic filters. Since the end of 2008 they are shipping their new "Halo-Free" filters.

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.

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.

Suitability

• 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 (astro modified): Very good, colour balance is near perfect
• DSLR photography (MC modified): Good, when used with an additional IR-block-filter
• DSLR photography (original): Good, colour balance shifted but contrast enhanced 
• Webcam / Video (Planets): Unsuitable
• Webcam / Video (Deep Sky): Very good, if light pollution is a big problem

Technical Data

• 97% transmission at 656nm (H alpha)
• 92% transmission at 486nm (H-beta)
• 92% transmission at 496nm (OIII)
• 92% transmission at 501nm (OIII)
• 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

How to read the above 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 filled with grey  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 of the eye 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 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 most important artifical emission lines are shown in orange. The artifical light pollution is dominated by mercury (Hg) and sodium (Na), which are used in nearly all streetlights.
   

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