A T-adapter is a coupling tube that connects your DSLR single lens reflex camera (with its lens removed) to your catadioptric telescope or spotting scope. It lets you take full disk pictures of the Sun and Moon, as well as pictures of distant objects here on Earth. One end of the T-adapter threads onto the rear cell of your catadioptric telescope (or onto the 1.25” focuser drawtube of some refractors and reflectors) in place of the star diagonal and eyepiece. The opposite end of the T-adapter has standard male 42mm x 0.75mm pitch T-2 threads that connect to an optional T-ring designed to fit your particular camera body. A slip ring on the T-adapter allows you to rotate your camera to align it horizontally or vertically for the best composition. A T-adapter can also be used to connect SBIG self-guiding CCD cameras to your scope.
All T-adapters need an optional T-ring to connect them to a camera.