UHC filter
A UHC filter ( U ltra H igh C ontrast filter ) is a mist filter for improving the amateur astronomical observation of gas mists . It is a narrow band filter that allows only a small part of the wavelengths of light to pass through, but blocks the light from street lights .
The wavelengths of light that are important in the observation of nebulae , the so-called H-beta line at 486 nm, the O-III lines at 496 nm and 501 nm as well as the H-alpha line with a wavelength of 656 nm, are on the other hand transmitted . In addition, light from the singly ionized sulfur, the S-II lines at 672 nm, is usually also let through. This allows the observer to filter stray light out of the city. The practical result is that the background around the target object becomes very dark. However, the light of the observed nebulae penetrates largely undiminished, which results in a greatly increased contrast and thus a better quality observation.
Most interfering light sources can be found particularly in the range from 540 nm to 640 nm wavelength. This part of the spectrum is completely blocked by a UHC filter.
UHC filters are attached to the telescope on the eyepiece side and screwed onto the inside of the eyepiece . They are usually available in 2 "and 1.25" mounts, depending on which eyepiece size is used. For digital SLR cameras , they are offered as clip-on filters for the lens and as clip-in filters for installation in the housing (in front of the reflex mirror).
Meaningful use
UHC filters are designed exclusively for deep sky observation and are unsuitable for planetary observation . The preferred area of application are those objects that emit light components in the O-III lines as well as in H-β. Furthermore, they are used with smaller optics up to about 20 cm opening for the observation of objects, which prefer to emit light of the doubly ionized oxygen O-III, but in which the use of the O-III filter (see nebula filter ) leads to one strong image darkening.