Fog filter
A nebula filter is an optical filter that is used in astronomy, especially by amateur astronomers , to make nebulae more visible in space. The mode of action of nebula filters is to suppress terrestrial scattered light , which makes it difficult to observe numerous astronomical objects in regions of high light pollution , while those spectral lines in which the observed objects shine particularly intensely can pass the filter.
The fog filters are based on the principle of interference filters (more precisely: dielectric filters). Several layers of transparent materials, usually metal oxides and fluorides , are applied to a glass substrate . The optical properties are determined by the individual layer thicknesses and refractive indices of the dielectrics used. These show slight variations from manufacturer to manufacturer.
A distinction is often made between broadband, narrowband and line filters, even if these categories are not clearly delimited from one another.
application
Nebula filters are used for observing planetary nebulae , galactic nebulae , supernova remnants and Wolf-Rayet objects . What all these object classes have in common is that, in contrast to galaxies , for example , they preferably emit their light in narrowly limited areas of the spectrum ( spectral lines ). Thus, these objects are sufficiently separated from the scattered light, which is usually continuously distributed over larger wavelength ranges ; this difference allows the use of filters.
Broadband and narrowband filters
Both broadband and narrowband filters transmit a selected range of the spectrum, but their design is different:
- Broadband filters allow wide areas to pass through and only block the area in which the disturbing lines of night sky glow and light pollution mainly occur. Examples of broadband filters are the deep sky filter and the light pollution reduction filter (LPR filter).
- With narrow band filters, almost all wavelengths are blocked and, if possible, only the narrow range from the H-β line to the [O III] doublet is allowed through, with some manufacturers also the H-α line . An example of a narrow band filter is the UHC filter .
Line filter
Line filters are very narrow-band filters which only allow certain spectral lines emitted by the object to pass.
Some examples of line filters are:
- H-β filter: spectral line of the H-beta line of hydrogen at 486 nm, mainly used visually for emission nebulae
- O-III filter: Spectral lines of the doubly ionized oxygen at 496 nm and 501 nm, mainly used visually for planetary nebulae , supernova remnants and Wolf-Rayet objects
- H-α filter: Spectral line of the H-alpha line of hydrogen at 656 nm, mainly used for photography / CCD astronomy for emission nebulae ( H-II regions )
- N-II filter: Spectral line of the singly ionized nitrogen at 658 nm
- S-II filter: Spectral line of the singly ionized sulfur at 672 nm, mainly used for photography / CCD astronomy for star formation areas .
The lines of S-II and H-α are in the red part of the visible spectrum . Since the sensitivity of the dark-adapted human eye is low in this spectral range, the corresponding line filters are only used for photography or with CCD cameras. However, there are also instruments available with a built-in H-α filter, so-called PSTs (Personal Solar Telescopes) , for observing the sun .