Main use of a CLS filter

The Astronomik CLS filter is the best filter to block annoying light pollution. Commonly referred to as a “light pollution filter”, it 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.

Astronomik CLS filters are also very useful for deep sky imaging with unmodified DSLRs. (For CCD cameras and modified DSLRs where the internal IR filter has been removed – the CLS-CCD Filter should be used).

The filters are optimised for use with telescope focal ratios from f3 to f15. 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 (a UHC-E or UHC filter is more suitable in heavy light pollution)
• 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

Available sizes:

We stock Astronomik CLS filters in 1.25″, 2″ and EOS-Clip sizes. Other sizes are available as special order items with order times of 2-3 weeks. Astronomik have recently changed their unmounted round filters to now incorporate a thin metal cell that helps to protect the edge of your filters from chips and cracks. These filters are available in 31mm, 36mm and 50mm sizes. Other diameters are available by special request with Astronomik to meet your requirements.

1.25″ (M28.5)

2″ (M48)

EOS-Clip Filter

SC Rear Cell (2″-24tpi)

T-Thread Cell (M42 x 0.75)

31mm Diameter Mounted Cell

36mm Diameter Mounted Cell

50mm Diameter Mounted Cell

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.

It blocks the light of mercury and sodium-vapor lamp spectral lines (street lights), while letting the largest part of 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, pass through the filter unhindered.