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Astronomy & Telescopes Glossary - T
Telecompressor - A telecompressor decreases the focal length and magnification of a telescope. It is commonly used in astrophotography to reduce exposure times. The opposite of a telecompressor is a Barlow lens.
Telrad - A popular brand of unit-magnification or single-power (1X) telescope finder that displays 3 red concentric circles in the viewfinder.
Totality - The part of an eclipse when the moon entirely blocks the Sun or the Earth's shadow totally blocks the moon.
Transit - When a smaller body passes in front of a larger one. The only planets that ever pass between the Earth and the Sun are Mercury and Venus.
True field - How much sky, in angular measure, is seen through an eyepiece.