page_title: H-alpha
section_title: Filter
hierarchical_section_title: H-alpha / Filter
caption_attribution_description: English: NGC 6888 (Crescent Nebula) and surrounding nebulosity in Hα (H-alpha) (3nm) imaged using amateur telescope. Русский: Эмиссионная туманность «Полумесяц» в созвездии «Лебедь» (NGC 6888) видимая через фильтр Hα (ширина полосы пропускания фильтра - 3 нм). Сфотографировано с помощью любительского телескопа Джефом Джонсоном (Jeff Johnson).
context_page_description: H-alpha is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha light is the brightest hydrogen line in the visible spectral range. It is important to astronomers as it is emitted by many emission nebulae and can be used to observe features in the Sun's atmosphere, including solar prominences and the chromosphere.
context_section_description: An H-alpha filter is an optical filter designed to transmit a narrow bandwidth of light generally centred on the H-alpha wavelength. These filters can be dichroic filters manufactured by multiple (~50) vacuum-deposited layers. These layers are selected to produce interference effects that filter out any wavelengths except at the requisite band.
  Taken in isolation, H-alpha dichroic filters are useful in astrophotography and for reducing the effects of light pollution.  They do not have narrow enough bandwidth for observing the sun's atmosphere.
  For observing the sun, a much narrower band filter can be made from three parts: an "energy rejection filter" which is usually a piece of red glass that absorbs most of the unwanted wavelengths, a Fabry–Pérot etalon which transmits several wavelengths including one centred on the H-alpha emission line, and a "blocking filter" -a dichroic filter which transmits the H-alpha line while stopping those other wavelengths that passed through the etalon.  This combination will pass only a narrow (<0.1 nm) range of wavelengths of light centred on the H-alpha emission line.
  The physics of the etalon and the dichroic interference filters are essentially the same (relying on constructive/destructive interference of light reflecting between surfaces), but the implementation is different (a dichroic interference filter relies on the interference of internal reflections while the etalon has a relatively large air gap). Due to the high velocities sometimes associated with features visible in H-alpha light (such as fast moving prominences and ejections), solar H-alpha etalons can often be tuned (by tilting or changing the temperature) to cope with the associated Doppler effect.
  Commercially available H-alpha filters for amateur solar observing usually state bandwidths in  Angstrom units and are typically 0.7Å (0.07 nm).  By using a second etalon, this can be reduced to 0.5Å leading to improved contrast in details observed on the sun's disc.
  An even more narrow band filter can be made using a Lyot filter.
caption: An amateur image of NGC 6888, using an H-alpha (3 nm) filter
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