Astronomik ASHA12nm1 - H Alpha Filter, 12 nm, 1.25"

This filter is suitable for photography of hydrogen nebulae from artificial light polluted areas, but also from places with dark skies. The contrasts between objects shining in H-alpha light and the sky background are extremely enhanced.

The H-alpha filter increases the contrast between hydrogen nebulae and the sky background. Due to the narrow half-width combined with high transmission in the H-alpha line region, the filter achieves a greater contrast enhancement than wider band filters. The half-width of 12 nm is optimally matched to the use of common CCD and CMOS sensors. The half-width makes it possible to use fast optics. In addition, the filter has a transmission of almost 100%, which cannot be achieved with narrower-band filters.

Even in the big city, shots with cooled cameras are usually limited by the dark current. Therefore, further suppression of the sky background by lower half-widths does not reveal more details in the object. Compared to very narrowband filters, the H-alpha 12nm filter then has the advantage that you can usually find tracking stars for almost all cameras with a built-in tracking sensor.

An example of an image taken from the outskirts of a large city (Munich) can be found under the field reports.

The useful range of application extends over a focal ratio of 1:2.0 to 1:30 for lenses and telescopes of all diameters.

Tips and further usage notes:

With H-alpha- OIII- and SII-filters you can create three-color images of emission line objects even from locations with very strong light pollution, as it is known e.g. from images of the Hubble Space Telescope.

In this case, a CLS CCD filter can be used to capture the luminance channel.

Achromatic refractors show relatively large stars due to the residual color error. If you use line filters like the ASHA12nm, the stars will be much smaller on the images.

Astronomik H-alpha filters are not suitable for solar observation!

Alternatives:

The 6-nm H-alpha filter is interesting when objects in very starry areas of the sky are to be imaged with short focal lengths. The number of stars in the image is typically halved.

Suitability of the filter:

Visual Observation (Land): Unsuitable

Visual observation (city): Unsuitable

Conventional film: Neutral, but very long exposure times

CCD and CMOS astrophotography: Very good, extreme contrast gain for H II emission nebulae

DSLR astrophotography (unmodified): Good, but much lower sensitivity in H-alpha than modified cameras.

DSLR astrophotography (astromodified): Very good, extreme contrast gain in H II emission nebulae.

DSLR astrophotography (modified with MC clear glass): Very good, extreme contrast gain with H II emission nebulae.

Webcam / Video (planets): Unsuitable

Webcam / Video (Deep Sky): Good, good contrast gain for bright objects

Technical data of the filter:

Transmission of over 97% at the H-alpha line (656 nm).

HWB 12 nm

complete blocking of all interfering wavelength ranges up to infrared

Homofocal with all other Astronomik filters

Thickness of the filter: 1 mm

Not sensitive to moisture, not aging, scratch resistant

Fine optical polished substrate material

The filter is supplied in durable packaging

The transmission curve:
[[kein Bild]]

On the horizontal axis, the wavelength is plotted in nanometers. 400 nm corresponds to deep blue, at 520 nm the human eye sees green, at 600 nm red. At 656 nm is the well-known "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 gray curve in the background shows the relative sensitivity of the night-adapted human eye. The sensitivity maximum is at 510 nm and falls symmetrically to the short and long wavelength regions. It can be seen that the night-adapted human eye cannot perceive any radiation in the H-alpha line at 656 nm, the sensitivity is 0%!

Photographic filters: The gray curve in the background shows the relative sensitivity curve of a typical astro camera sensor.

In orange are shown the most important emission lines contributing to artificial sky brightening. These are the lines of mercury (Hg) and sodium (Na) used in most streetlights and neon signs.

The main emission lines of gas nebulae are shown in green. These are the lines of hydrogen (H-alpha and H-beta) and the lines of doubly ionized oxygen ([O III]) and singly ionized sulfur ([S II]). The square brackets indicate that these are forbidden lines.

Safety informations: PDF Download