Used: Astronomik ASCLSCCD50R - CLS Filter, 50 mm, round, unmounted

The Astronomik CLS filter is suitable under light-polluted skies for DSLR Cameras, which have been remodeled for astronomical use. The filter enhances the contrast between all deep-sky objects and the background.

The CLS Filter enhances the contrast between astronomical objects and the background. Due to the wider transmission curve compared to UHC filters, a greater amount of light will pass the filter. Stars will be less dimmed. This filter has been optimized to block as much spurious light as possible and simultaneously provide the best performance for ´useful´ light. A good filter for DSLR, CCD, CMOS, and film b/w-photography as well as observation of deep-sky objects with telescopes or photo lenses of all aperture f/3 and above.

Main use:

The filter blocks completely emission lines of artificial light sources like streetlights (e.g. sodium and mercury vapor) as well as the airglow. All ´important´ emission lines as well as the spectral range of the night-adapted eye are beeing passed. The supplementary IR-blocking layer allows the use for DSLR and Webcams without an integrated IR-blocking filter.

Other uses:

Easier resolution of Double Stars.

With the EOS-Clip model, photography with DSLR cameras is feasible even unter extreme light-polluting conditions without shifting the white balance.

The 1,25" and 2" threaded models can be used for observations from light-polluted areas.

If you plan to create color images from emission line data, the CLS filter is a great choice for the Luminance channel.

Alternatives:

CLS: For cameras with an integrated IR-blocking filter or for visual observation.

Suitability:

Visual observation (dark skies): Good, to reduce light pollution by sodium-vapour and low-pressure mercury-vapour lamps (streetlight)

Visual observation (urban skies): Reasonable, an UHC-E or UHC filter is more suitable

Film photography: Very good, colour balance is nearly perfect

CCD and CMOS photography: Very good, optimized rejection of light pollution

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): Very good, colour balance is near perfect

Webcam / Video (Planets): Unsuitable

Webcam / Video (Deep Sky): Very good, if light pollution is a big problem

Technical Data:

95% transmission at 486 nm (H-beta)

95% transmission at 496 nm ([O III])

95% transmission at 501 nm ([O III])

97% transmission at 656 nm (H-alpha)

pass from 450 to 520 nm and from 640 to 690 nm

Parfocal with other Astronomik filters

Glass thickness: 1 mm

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

Note on the protective ring:

Astronomik delivers the round filters from 31 mm diameter with a protective ring at the edge. The corresponding declarations of the manufacturer are available for download as PDF.

Filter transmission curve:

The horizontal axis is the wavelength in nanometers (nm). 400 nm is deep blue, at 520 nm the human eye senses green and at 600 nm red. At 656 nm 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 ~510 nm 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 656 nm is 0% at night!

Photographic filters: The grey line in the background shows the sensitivity of a typical astro camera 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), double ionized oyxgen ([O III]) and ionized sulfur ([S II]). The square brackets indicate that these lines are forbidden.