Astronomik ASHA6nm2 - H Alpha Filter, 6 nm, 2" mounted

The Astronomik 6 nm H-alpha filter is an extremely narrow emission-line filter for astro photography. The filter lets the H-alpha light of emission nebulae pass and blocks nearly the whole remainder of the spectrum where the Sensor is sensitive.

The full width at half-maximum (FWHM) of 6 nm is optimized for a maximum contrast between the (H-alpha emitting) object and the background. The contrast gain is much higher than with our 12 nm emission line filters, but you won’t see as many stars in the resulting images.

The design of the transmission curve makes it possible to use the filters with instruments from 1:3.75 to 1:15. Selected filters for faster system are available upon special request.

All transmission curves of the 6 nm filter are measured individually. Because of this procedure you can get the optimum filter for your telescope: If you place a direct order please specify the focal ratio of your instrument and we will select the best filter for your setup. So you may get the best filter without paying for an A+ filter.

Main use:

The Astronomik H-alpha filter (6 nm version) increases the contrast between objects, in this case between the H-alpha emission line and the skyglow background in the same way as the 12nm version. The gain in contrast is much higher due to the smaller FWHM. The 6 nm H-Alpha filter is the right choice for extremely dim objects, even in the milkyway, and for observations from heavily light-polluted places. The Astronomik 6 nm H-alpha filter completely suppresses the emission lines of artificial lighting (mercury (Hg) and sodium (Na)) and skyglow. Transmission losses and chromatic distortions, which arise with other filters, only occur with Astronomik filters when extremely bright aperture ratios come into play. Selected filters for faster systems are available upon special request.

Other uses:

Using the Astronomik 6 nm H-Alpha together with the 12 nm H-Alpha gives you the ability to have a maximum contrast in the HII objects and to keep the stars in the final image.

Achromatic refractors still have chromatic aberrations which make the star images bigger. With lines filters like this one, star images will shrink.

The Astronomik 6 nm H-Alpha filter may NOT be used for solar observation.

Alternatives:

If you plan to do (false-)color imaging with other emission line filters, you should get our filters with 12 nm FWHM. Images taken with filters with differing FWHM are very difficult to combine into the final image.

Suitability:

Visual observation (dark skies): Unsuitable

Visual observation (urban skies): Unsuitable

Film photography: Unsuitable

CCD and CMOS photography: Very good, huge contrast enhancement at H II-emission nebulae

DSLR photography (original): Good, reduced sensitivity in the H-alpha band

DSLR photography (astro modified): Very good, huge contrast enhancement at H II-emission nebulae

DSLR photography (MC modified): Very good, huge contrast enhancement at H II-emission nebulae

Webcam / Video (Planets): Unsuitable

Webcam / Video (Deep Sky): Unsuitable

Technical Data:

Transmission of over 80-97% at the H-alpha line (656 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

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.