To connect an SLR camera to your telescope, you’ll need two accessories: a camera T-ring and a T-adapter. T-ring is also known as T2 mount, and T-adapter is sometimes known as T-mount. This system is a worldwide standard 42mm thread to connect cameras to other devices.
A T-adapter attaches to the scope and a T-ring attaches to your SLR camera. A T-adapter with a 1.25” eyepiece barrel is very versatile and slips into the eyepiece drawtube of most telescopes. Other types of T-adapters with threads are specific to certain types of scopes. (Specialized accessories like a radial guider or tele-extender have T-threads that take T-rings, so they act like specialized T-adapters). For example, a T-adapter has been purposely designed to connect various SLR cameras to the Nipon 25-125x92 spotting scope.
The threads on the T-adapter fits into all T-rings. Since each brand of camera has its own specific thread size or bayonet type, you need the proper T-ring. Thus Canon has its T-rings (two of them); Nikon has its own T-rings; Minolta has their own specific rings, and so on. Some T-rings are available here for most popular brands, such as Sony, Olympus 4/3, Olympus Micro 4/3, Pentax K, in addition to those mentioned earlier.
To attach SLR cameras to your telescope, first, remove the visual accessories you are using – eyepiece, diagonal, visual back. Then either screw on or insert the correct T-adapter for your scope and the type of photography you want to do.
For example, to use the T-adapter with the 1.25” barrel, remove your SLR's lens and attach the T-ring to your camera, then the T-ring to the T-adapter and slide the 1.25” barrel into your eyepiece holder. Add a shutter release and the camera is now ready for prime-focus photography. Your telescope acts as a platform and can be used to provide tracking if the scope has been polar aligned. Tracking may not be needed for very short exposures of the sun, moon and planets.
Important Note: For some Newtonian telescope models, the focal plane of the telescope may not be far enough from the tube to allow you to focus with the T-adapter/camera setup you are using. Try using a T-adapter with a built-in Barlow lens (with the lens at the forward end), such as this Nikon SLR adapter, to extend the focal plane to reach your camera’s imaging plane. Another way that will achieve infinity focus with almost any Newtonian or other telescope is eyepiece projection directly into your camera with a large 32mm, 40mm, or 10-30mm zoom eyepiece. Some eyepieces have even got T-threads under the rubber eyecup, so simply thread it into your camera T-ring and then attach to your camera and put the assembly into your scope’s eyepiece drawtube.
More information can be found from this link: Digiscoping