Filter

Filter

Filters are used in astrophotography to increase contrast by allowing only light of certain wavelengths to pass through. Many objects (e.g. emission nebulae, planetary nebulae,...) emit or radiate only light waves of a certain spectrum. The electromagnetic wave spectra depend on the chemical elements in the object or in the gas surrounding the object. By using narrowband filters, which only allow a very narrow wavelength range to pass through to the chip, interfering, superimposed signals can be filtered out and a detailed image is produced.

Since narrowband filters let through only very little light, the single exposure time increases enormously. Pure narrowband filters are not suitable for color cameras, since the Bayer-matrix would only let red light through on every 4th pixel, for example. An exception is a Duo-Narrowband filter that passes light at 500nm and 656nm. At 500nm the green and blue pixels are activated, at 656nm the red ones.

Starlight when using a Bahtinov-mask, taken with a Duo-Narrowband filter andcolor camera

If these narrow band images are extracted (Duo Narrowband with color camera) or directly created with a monochrome camera, the image is black and white and by the astrophotographer a color is assigned afterwards. There is no limit to the artistic freedom, but mostly the colors of the corresponding wavelengths are used.

From galaxies and star clusters light comes to us from the entire visible spectrum. Here, a filter should only be used in exceptional cases, when additionally details for a wavelength range are to be displayed.

Other objects where filters should not be used are reflection nebulae. They reflect light from nearby stars and therefore show the full light spectrum of that star.

The most common filters are:

• • H-beta-filter (486 nm)
  • O-III-filter (496 / 501 nm)
  • H-alpha-filter (656 nm)
  • N-II-filter (658 nm)
  • S-II-filter (672 nm)
  • CLS-filter (mostly broadband with some selected blocked areas)
  • UHC-filter (mostly narrow band with larger blocked areas)

For planetary images, wider band color filters or IR filters are often suitable to increase surface contrast.

For areas close to cities, very broadband and also somewhat narrowband filters, so-called light pollution filters (e.g. UHC, CLS) are often still used, which are designed to filter out interfering light from artificial lighting (e.g. sodium lamps). These are also very suitable for visual observation.