A telecompressor is an optical element which amateur astronomer

Amateur astronomy

Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...

s use to reduce the focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 of a telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

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Mainly used for astrophotography

Astrophotography

Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...

, the telecompressor gives a wider angle of view, a sharper picture, and reduces the exposure time neededhttp://www.astropix.com/HTML/I_ASTROP/EQ_TESTS/VIGNETTE.HTM. These are useful when taking pictures of nearby large objects, such as nebula

Nebula

A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

s. The effects and uses of the telecompressor are largely opposite to those of the teleconverter

Teleconverter

A teleconverter is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens...

 or barlow lens

Barlow lens

The Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal ratio of an optical system as perceived by all components after it in the system...

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