Astronomik H-alpha 6 nm 2" filter (Astronomik, SKU: 10212555) is a narrowband filter for imaging nebulae emitting light from hydrogen H-alpha at 656 nm. Its 6 nm full width at half maximum increases the contribution of the selected emission relative to the sky background, both at dark sites and under light pollution.
The filter suppresses light outside the useful passband, from UV to IR. It can be used for monochrome imaging or to build composites from exposures taken through several emission-line filters. Exposure time and channel balance depend on the brightness of the individual lines, camera sensitivity and sky conditions.
The 6 nm passband suppresses background and starlight more strongly than the 12 nm version. It is useful for faint nebulae, crowded star fields and cameras with low dark current. A narrower passband requires particular attention to the optical system’s focal ratio.
The filter has a 2" cell with an M48 × 0.75 thread. It fits accessories with a matching filter socket; the nominal cell size is not the diameter of the clear glass aperture.
The H-alpha filter is not a solar filter and must not be used for observing or imaging the Sun.
• passband: H-alpha, 6 nm full width at half maximum
• suppression of unwanted out-of-band light from UV to IR
• recording of one emission-line channel for later combination
• coating: Astronomik MFR
• durable coatings resistant to scratching and moisture
• filter: Astronomik H-alpha 6 nm 2" filter
• protective filter box
• manufacturer: Astronomik
• manufacturer product code: 10212555
• type: H-alpha narrowband emission-line filter
• full width at half maximum (FWHM): 6 nm
• guaranteed transmission: over 90% at the H-alpha line at 656 nm
• typical transmission: 93% at the H-alpha line at 656 nm
• out-of-band blocking: from UV to IR
• coating: Astronomik MFR
• recommended focal ratio range: f/4–f/∞
• cell size: 2"
• thread: M48 × 0.75
• glass thickness: 1 mm
• material: finely polished optical glass
• optical quality: diffraction-limited
• coating resistance: to scratching, moisture and ageing
• parfocality: with standard Astronomik filters using 1 mm glass
• quality control: individual measurement of spectral response
Selective H-alpha imaging helps reveal nebular structure with a reduced sky-background contribution.
