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T-Adapter - T-threads are a common thread size used to attach cameras to telescopes.  A T-adapter is specific to a particular telescope on one end and has universal T-threads on the other, allowing a T-ring to be attached.

T-Ring - A T-ring is an adapter attaches to a particular SLR camera and has universal T-threads on the other side to attach to a T-adapter.

Telrad - One of the most popular brands of reflex finders, a Telrad projects a red bull's-eye onto a head's-up display, making finding objects much easier than a standard optical finderscope.

Tracking Rate - This is the speed at which a telescope mount moves to track celestial objects.  For most objects this is called the sidereal rate, which is the rate of Earth's rotation.  Other common rates include solar and lunar tracking rates which adjust the sidereal rate slightly to compensate for the motion of these objects with respect to the background stars.

Transparency - This is a measure of the clarity of the atmosphere.  The better the transparency, the clearer the sky is and the better you can see deep-sky objects.  Transparency is often confused with seeing, which is the steadiness of the atmosphere and is important for high-power planetary observing.  Good transparency and seeing are often mutually exclusive, with the best seeing often coming on nights of poor transparency.

Triplet - An objective lens that uses three elements is called a triplet.  Triplet lenses are apochromatic.

True Field - This is the actual area of sky seen through an eyepiece.  It is a function of magnification and apparent field, a property of the eyepiece design.  Typical true fields for common telescopes range from about 0.25° to 2°.

Truss Tube - Instead of a solid tube containing the optics, some telescopes use multiple truss poles to support the front and rear sections of the telescope (which contain the optics).  This is especially common in very large telescopes (usually larger than 12" aperture) where a single large optical tube would not be easily portable.  Many large Dobsonians use truss poles, as do some Ritchey-Chrétien telescopes.

Twilight, Astronomical - Twilight is normally thought of as just the time between sunset and when it gets dark, but there are three strictly defined twilights:  civil, nautical, and astronomical.  Civil twilight lasts from sunset until the sun is 6° below the horizon.  Nautical twilight ends when the sun is 12° below the horizon.  Astronomical twilight ends when the sun is 18° below horizon.  This is when the sky is considered to be fully dark.  How long the sun takes to get 18° below the horizon depends on your latitude and the time of year, but astronomical twilight generally ends about 90 minutes after sunset and begins about 90 minutes before sunrise.

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