A T-ring is the coupling ring that is used to connect your 35mm single lens reflex camera body to all telescope and terrestrial spotting scope photographic accessories – camera adapters, T-adapters, off-axis guiders, flip-mirror systems, tele-extenders, etc.
Except for Questar T-rings (that are used exclusively with Questar camera coupling sets and are listed separately), each T-ring is internally threaded on one side to attach to the Japanese-standard T-2 threads (42mm x 0.75mm pitch) that is used on all T-adapters, camera adapters, etc. The other side of the T-ring has the appropriate external thread pattern or bayonet mount to connect to your particular camera body.
Three small recessed screws in the body of the T-ring (accessible only by a small jeweler’s screwdriver or hex wrench) allow you to align your camera horizontally or vertically for the best composition with those very few camera adapters that do not have an adjustable slip-ring. Once you have set the camera orientation in this way, these screws must be retightened very carefully to keep the T-ring components from separating while in use, allowing the camera to fall to the ground.
The suppliers of these T-rings may vary, depending on product availability. Suppliers may include, but are not limited to: Bushnell, Celestron, Kalt, Meade, Photoco, or Swift. In all cases, while the appearance may vary slightly from brand to brand, the mechanical performance of the T-rings will be identical.