Telecompressor

A telecompressor is an optical element which amateur astronomers use to reduce the focal length of a telescope.

Mainly used for astrophotography, the telecompressor gives a wider angle of view, a sharper picture, and reduces the exposure time needed [http://www.astropix.com/HTML/I_ASTROP/EQ_TESTS/VIGNETTE.HTM] . These are useful when taking pictures of nearby large objects, such as nebulas. The effects and uses of the telecompressor are largely opposite to those of the teleconverter.